Since Sametime still does not support Internet Site documents, you have to do a little hack:
1. Disable Internet Site documents in the server doc for your Sametime server
2. Find the SSO document you created, copy and paste it in to the same view. You should now have two of these, both with the same key
3. Edit the copy you made and clear out the "Organization" field, and rename it 'LtpaToken' and save it. This will make it disappear from the Internet Sites view, and appear in the old Web Configuration view.
4. Edit both server documents to make sure they point to the correct Multi-Server SSO document (DWA server to the original, Sametime server to the new LtpaToken document).
5. See this document
if you want to name the SSO document something other than LtpaToken. Also see this document
for the complete steps to DWA 7/ST 7 integration.
If you ever bring up a QuickPlace server you'll need to use that new SSO document you just created. QP supports the use of Internet Site documents, but then you lose Sametime awareness, so you have to switch back to using Web Configuration documents.
Lotus - if you're reading this - I don't care if it is documented, it's still a hack. We're on our second major release with Internet Site documents, and Sametime and QuickPlace still don't fully support them.