Try taking the assets.tdx and assets.bku files out of the Trainz directory and restart CMP. It may take a while to reload. By removing them and not deleting them, you can always replace them if something horribly goes wrong. (Natvander).
Once you restart CMP it should rebuild your contents database. Could take awhile to rebuild.
(Your assets.tdx or assets.bku could be corrupt, and that's possibly why no additional assets are showing up).
If that doesn't work, just put the orginal .tdx & .bku files back to the directories you tranferred them out of.
Cheers... Mac