Hi, I found this answer on apple forum see if it works for you.
Lion does not support PowerPC applications - the Rosetta engine is no longer available , but the older JamPacks have installers that only run on PowerPCs, and so you cannot install those on Lion easily.
If you have the loops available from a backup, the best way to install those is to drag the loops onto the Loop Browser panel in Garageband; then GB will install them properly in the library, but you also will need to install the corresponding instruments in your instruments library. The instruments need to be installed in
Your system volume -> Library -> Application Support -> GarageBand -> Instrument Library
BTW: The easiest way to install all JamPacks at once for OS Lion would be to buy MainStage 2 from the App Store. It will install all JamPacks and the Instruments plus many additional loops from Logic Pro and is much less expensive than any of the JamPacks.