PJROBAR  Documentation is included, at least as downloaded from MacUpdate. Look for a file named "F.A.Q" (Frequently Asked Questions) in the folder produced after the download is expanded.

The application does not install anything in System Preferences. Its GUI shows up in the menu bar. (From whence you can access its preferences via a menu.)  
(Version 2.2.2)

NOTAFOOL  Sorry, but the newer version is just as bad the previous one. It simply does NOT - repeat NOT - launch on my computer, (Details of my computer and OS are in my first posting). This is extremely disappointing. Quite frankly, I no longer have ANY confidence in this application at all and shall NOT be trying it again. It appears to have been quite inadequately tested on the VERY machines that it purports to have been written for, and is bug-ridden. It has been released far too prematurely and simply does NOT work. BEWARE !!  
(Version 2.2)

NOTAFOOL  This latest version - purporting to have been released today (21 April 2009) - is no better than any of the previous version I have tried. It still totally refuses to launch or do anything whatsoever. It remains a total LEMON as far as I am concerned. I have had quite a bit of correspondence with the Developer and have sent him crash reports, logs etc. So far he has no idea why it doesn't work for me. But it certainly doesn't. And check out his website. Many, many people are having all sorts of problems with it. It is simply NO good.  
(Version 2.2.2)

ST00P!D M0NK3Y  Try this...

Drag smcFanControl to the Dock manually. Then when you have it on your Dock, right-click (or control-click) the icon and select "Open At Login" and then restart the computer. It wasn't launching at startup for me either until I did this manually. See if it works for you.  
(Version 2.2.2)

NOTAFOOL  I did exactly as you recommended STOOPID MONK3Y. Absolutely NO difference whatsoever. The dopey application STILL totally refuses to launch. NOTHIng. ABSOLUTELY NOTHING. Not even the remotest hint that it is launching. I have given up on this dopey application now. It is a total LEMON as far as I am concerned. I have had correspondence with the Developer, who appears to have absolutely no idea why it will not launch even though I have sent him crash reports etc. I have now trashed this irritating application and intend having NOTHING to do with it. It appears to have more bugs in it than an entomologist's laboratory.  
(Version 2.2.2)

ST00P!D M0NK3Y  Dude, it's working perfectly for me and thousands of others. Don't know what the issue is and I wish I could tell you, but there must be something funky in your system from preventing the launch of this app. If even the developer can't help you, I don't think anyone else can. Maybe wait for a newer release or just ditch the program altogether. The updates seem to be far and few between though so you might be waiting a while.  
(Version 2.2.2)

M. LUTHER  Works fine for me. And if a stoopid monkey can get it working, then its gotta be something funky with your system.  
(Version 2.2.2)

NOTAFOOL  Every other application on my computer works absolutely fine. I have run both Disk Warrior and Drive Genius over it and both give it a 100% clean bill of health. If my computer is, in fact, "funky", then how come that doesn't show up in any way at all except with THIS particular application ? Never mind. I am quite content to do without SMCfancontrol. It's not such a big deal. But I'm certainly surprised that the Developer cannot offer any suggestions as to why it refuses to work on my computer, even though I have sent him Crash and Log reports.  
(Version 2.2.2)

TERMINALMAN  Note that this is advertised as "officially supporting" Intel-based laptops only. The iMac seems not yet officially supported, which could account for your difficulties.

There were a number of different Intel iMac versions released (three as of May 2009, I believe) and Apple likely changed the fan control interface with each version just as they seem to have done with the new laptop versions.

It is likely that the author does not have access to all of the Intel iMac variations, so complaining IN CAPS that it does not work for you is a bit over the top IMO. Particularly for freeware/donationware.

This is a useful app, works perfectly on three (different) Intel Mac laptops in my family, and will hopefully soon support all the different "respins" of the Intel iMac.

Honestly, software is not so trivial to write when the interfaces you're using "evolves" over time. Been there, done that, got the grey hair to prove it.  
(Version 2.2.2)

MAYERJS1  I was able to get the application to run, but certain aspects of it were unusable. The preference screen would come up blank, other menu items wouldn't appear at all. I removed the application from my system.

Running 10.5.6 on a MBP Core2DUO with SMC version 1.12f5  
(Version 2.2)

NOTAFOOL  I ended up removing the application as well. Very disappointing indeed !  
(Version 2.2)

GIANT AMOEBA  I have an intel-based iMac and was expecting the program to open a new window. It didn't, but then I looked up at the top right of my screen (near the day/time stamp) and there was a temperature and RPM reading.

Click that and the program displays it's options.

It works fine for me. :)  
(Version 2.2.2)