Installation and setup was very easy, but there is zero documentation (thats the reason for just 4 stars). I guess the lack of documentation is okay, because the app is still in beta and their support is very responsive (they replied literally within seconds to my email).

Installation and setup is like this:
- download the app
- unzip (doubleclick the zip file)
- doubleclick the unzipped prefpane
- select "install only for current user" (got this hint from their support)
- the app/prefpane starts
- enter your emailadress, a password and a password confirmation
- after a few seconds you get an email with your activation link - click it
- activate "always synchronize" in the prefpane
- click "synchronize now" in the prefpane
- ready

of course the app isn't very helpful if you don't install it on another computer.
so do the same on another computer (i tried it on my imac and my macbook). of course you shouldn't create a second account, but use the one you created on the first computer.

their support told me, that installation is easier if you decide which computer has your "master" addressbook and install on this system first, sync with the server, than delete the addressbook on the "slave" computer and install fruux there, too.