In order to get a Sprint Tour to work on AT&T's network fully you need:
1.) It to be unlocked (check!)
2.) Its PIN needs to be released from the Sprint BIS network (Can you get email on the device? If you can then it's already been released. If not, contact Sprint customer support to do so)
3.) You need to manually add the AT&T Service Books (the browser and MMS service books apparently) via the Desktop Manager.
Here's the BBOS How-to (It's done for a BlackBerry Storm but the same principle applies to the BlackBerry Tour)
The link to the AT&T service books in the above link is dead so I've attached the needed AT&T service books in this post as a zip file.
Keep in mind, this is somewhat of a hard/complicated procedure as your trying to use a CDMA BlackBerry on a GSM network which is why all these work-arounds are necessary.
Hope it goes well.