First, I would note there are things in the v11 Data Admin tool which won't work at all with a v10 DB since v10 db's don't have the same functionality of a v11 db.
Having written that, Progress platforms are designed to allow the clients to connect to databases one version ahead of the current client version via a client-server connection to support upgrading a database and then migrating the clients later.
I don't think it works the other way (a client can connect to a database one version back). What I'd suggest you try is connect to the db client server, and see if that helps.
If that doesn't work, then talk to your Tech Support resource, or check out the PSC KB:
http://knowledgebase.progress.com/progresskbsearch#
Most likely, you're out of luck and will need to upgrade the db to version 11 in order to use the version 11 Data Admin tool.