Currently Being ModeratedJan 27, 2013 2:35 AM (in response to Sgatscha)
do you have the same Mac OS X version? It can be that the location of some loops and instruments changed between versions.
And how do you save the GarageBand project? Have you tried to save it as an archive project?
That will copy all the loops you used and instruments into the project for easier sharing.
To be able to save as an archive, you may need to edit the song a little - otherwise GarageBand will not save again and the "archive" option will be grayed out. Use "File > Save as" and in the File Chooser panel check the button "Archive project".