Johann,
it doesn't work.I already have tried this before.Can it be because my imac is an intelmac and my ibook a G4?
I doubt it. The preferences file is just an XML file which is cross-platform.
When you say it doesn't work, what exactly do you mean? If you replace your current com.camerabits.PhotoMechanic.plist file from your /Users/<your user name>/Library/Preferences folder with the file from your other system then *something* has to change. There is no way for it not to change.
Maybe you're only copying the one from /Library/Preferences/ which contains only your registration information...
-Kirk