This one is simple but sometimes the simple ones frustrate you right ?
I couldn't for the life of me find any settings or anything in Linux Mint 4.0 Daryna to configure Compiz (Beryl), so here's what I did (I'm running GNOME, so this might not work with KDE but I don't know so it's worth a try)
1) Open Synaptic (Package Manager)
2) If you don't already have them, grab the compiz-plugins, compiz-fusion-plugins-main and the compiz-fusion-plugins-extra (optional)
3) Get the compizconfig-settings-manager. This is what will allow you to enable / customize your plugins.
4) Once everything is installed, you should have a new menu entry under Preferences called Advanced Desktop Effects.
5) Enjoy! (one guy put a Doom3 screenshot as his Skydome, to make his desktop cube look kind of like it was replacing the Soul Cube. Very clever... heh)