Mathew:
Go to your REST service directory in a browser and sign in.
Find your map service of interest and click on it to see the REST page for this service.
For "View In:" choose "ArcGIS Explorer"
When you click this link, you will be able to download an NMF file with this map service in the table of contents. Download the file, double-click it to open it. You will be challenged for a UN/PW. Add a UN/PW and the map will open with you map service. You can now "Share" the map service with other AGX users by right-clicking and choosing Share as an NMC file or share the map with others. Be sure to set check ON the Write authentication information for secure service into the map document in "Map Properties" | "Data Access"