Mike,
Without knowing which OS you're running on each computer it is a bit of a challenge. However, if I had to guess at the issue, it would be that you are running 10.6.X on the older Mac and 10.7.X on the laptop. File handling is a bit different with 10.7, so placing the library files and XML files just in the common location may be the culprit. Finale should automatically copy these files to your user location, but it doesn't always do this. You can check this by going to Preferences>Program> Files and finding out where things are placed and what is in each location. Let me know if this helps and which OS is on each computer.
Without knowing which OS you're running on each computer it is a bit of a challenge. However, if I had to guess at the issue, it would be that you are running 10.6.X on the older Mac and 10.7.X on the laptop. File handling is a bit different with 10.7, so placing the library files and XML files just in the common location may be the culprit. Finale should automatically copy these files to your user location, but it doesn't always do this. You can check this by going to Preferences>Program> Files and finding out where things are placed and what is in each location. Let me know if this helps and which OS is on each computer.