GREGW  Over 5 months later and I'm seeing this comment for the first time. The user never contacted me and hasn't provided enough information for me to help diagnose the problem. Perhaps this was an April Fool's joke.  
(Version 1.3)

GREGW  I can help you get it working if you'll contact me directly. I suspect you've just not put the plugin in a correct folder.  
(Version 1.2.2)

GREGW  I know about the shadow. The dock tile is by nature restricted to a 128-pixel square and I figured most people would prefer as much detail as possible on the image over a few extra pixels on the (rather subtle, IMO) shadow. I may tweak it during the next update, though.  
(Version 1.1)

GREGW  Just pointing out that as of v1.2.1 the skewed image has been adjusted to avoid clipping the shadow.  
(Version 1.2.1)

GREGW  Make sure that DockArt is the selected visualizer plugin, and then choose the Visualizer Options menu item. (Specific locations of these vary from version to version of iTunes.)  
(Version 1.0.8)

SIBELIUS  Thanks Greg - but I'm running iTunes 6.0.4 and can't find any visualiser options, even after selecting Dockart! Any ideas?  
(Version 1.0.8)

GREGW  It's been a while since I've seen iTunes 6, but I believe that the way to get at the visualizer options was to select the visualizer you wanted to configure and then actually activate the visualizer functionality. Once active, there was a button in the upper-right corner that would invoke the settings dialog.  
(Version 1.0.8)

SIBELIUS  No luck with that. Activating the visualiser with DockArt selected just brings up a black screen.

I also tried iTunes Help, and found this > "with visualiser selected, click the options button if available". It wasn't.

Looks like the skew effect only works with iTunes 7, but I guess I can live with that!  
(Version 1.0.8)

GREGW  "No luck with that. Activating the visualiser with DockArt selected just brings up a black screen."

DockArt's "real" visualizer is only a black screen because all it does is change the icon. It's not really meant to be selected as your primary visualizer.

"Looks like the skew effect only works with iTunes 7, ..."

So you're saying that when you select DockArt as your visualizer and then activate the visualizer there's no button in the window that says "Options" or "Settings" or something like that? Something's very strange on your setup if that's the case; iTunes has supported plugin settings dialogs at least back into the 4.x era, and I believe much further. (DockArt got its first user pref about 2 months before iTunes 5 was released.)

Here's a screen shot of iTunes 6: . What do you see in the upper-right corner in your copy of iTunes when DockArt is selected and the visualizer is turned on?  
(Version 1.0.8)

SIBELIUS  I've got it now - visualiser was set to full screen, which is why the options button did not appear!   
(Version 1.0.8)