Did you download an existing template from Maintenance/Data Maintenance/Document Templates, update it, and load it back in at the same location?
Once you do that, you should be able to preview the document from the Document Templates module or the PO module after you select the template and save.
There is a web video demo of how it document templates work in the Help file. If you are still having trouble I would contact Technical Support (800-892-0449).