Was the card formatted?
The TX doesn't come with a built-in file viewing or file management application for SD cards. Depending on the type of files, you should be able to view those files (or lists of them) with the application that is appropriate to those files...i.e. Documents with Documents To Go, music with PocketTunes, etc.
Also, some applications will look for files in particular folders. Music files should be in the /audio folder, for example. When you format the card in the Palm using the Card Info application, it creates these folders automatically.
But, you want to be able to view the files with something like a Windows Explorer app, right? The free answer is something called FileZ. Google the name and you'll find it...maybe at www.freewarepalm.com
or other sites. It does the job and it's free. But for something that's really the cat's meow in Palm file viewers, check out Resco Explorer. It does much more, including a .zip application, and other things. But it's not free. Check it out at www.resco.net