Apologies if I'm misunderstanding your post, and please don't take offense toward me being specific, some needing the same answer may not know what to do:
To start compiz at bootup, you can right click on the desktop, create new link to application. Give it whatever name and icon you want, it doesnt really matter. Go to the 'application' tab, set the command to be fusion-icon
Move this file you just made to /usr/share/autostart with root or sudo rights, either:
cp ~/Desktop/whateveryounamedthefile.desktop /usr/share/autostart
or if you have sudo rights,
sudo cp ~/Desktop/whateveryounamedthefile.desktop /usr/share/autostart
or just cut n paste it in /home/YOURNAME/.kde/Autostart (i would suggest /usr/share/autostart).
This will start the compiz fusion icon at startup. Right click its icon in the system tray, select window manager "compiz". That should start it fine. If you reboot with this setting, it will probably save it that way for the next boot.
Next question: do you have the cube enabled? You can right click the icon again, and under the settings manager check the box besides Desktop Cube. You might need a reboot for it to work if it doesn't work right away (which it usually does, just sometimes it wont).
I hope that helps..