TS2010/Duches/SnC - copy the relevant .ja files to TS12/Builltin, then add the serial numbers into the Trainz Options box, click OK, then do an extended database repair.
Note - If you install SP1 or purchased it recently, any .ja based addon packs you have installed (apart from TS2010) will become invalid and will have to be redownloaded using the ingame interface.
Local files - use Import Content function in Content Manager and point it at the relevant Local or Original folder.
Shane