Are you trying this on one of the new 2010 MBPs? That's what I am having the issue on - it's running 10.6.3 (pre-installed) and I believe it's either the OS or the machine that has something to do with Jam not working.
As I indicated above, I've tried a few methods to get it to work, the most recent was connecting my 2010 MBP in Target mode to an Intel iMac (successfully running Jam on 10.5.8) and installing Jam again via the original dmg file. Installation was successful but Jam still won't launch - it just bounces in the dock for a few seconds and quits. The .plist files are there & where they belong and seem to be proper format but the Roxio Jam Preferences file differs in that it appears as a Microsoft Excel file on the 2010 MBP as opposed to a Microsoft Entourage file on the iMac (which must be the correct format.) I also tried copying the existing iMac Jam + prefs. with the same results.
If it's 10.6.3 that's creating this change, I've considered downgrading back to 10.5.8. but I'm not yet sure that these new MBPs can revert and run 10.5.8. I know the generation of MBPs prior to the April 2010 upgrades can and I guess Jam works fine on them. So, if the 2010 MBPs will run 10.5.8, then, in theory, Jam should work fine. However, if it turns out to be the processor instead that has created this situation, then I seem to be out of luck.
This is one of the few programs that wouldn't work when I migrated over and I'm really disappointed because I've used it constantly for years. My research continues and hopefully someone here can help.