I just tried loading the files from the website to my 996 and they worked just fine. Make sure you open the system first (after unzipping them of course) and that you attempt to open the .usd files and not the .m96 ones. (The .m96 are for ARC996, the .usd are for UASD)
If you wish to enter the systems manually then set it to "MOT TYPE2/P25" for the System type, then add Sites, making them "P25". You can enter in just the control channels, but be advised that not all the control channels have been identified so to be safe you can enter in all the known channels for the site, chances are that if we missed a control channel it is already being used as a voice channel.
As for TGID's, you don't need to enter all of them, just assign names to the ones from your area, and you can start locking out the ones from areas you are not interested in.