Because of other items being grayed out, I assume that you are not connected as the sde user or an administrator or owner to that database.
From ArcGIS Managing Schema Locks Help Page, it appears that locks are available to the sde user (and my guess would be anyone else that is listed as an 'owner' to that database):
Clicking the Refresh button updates the list with the most current connection details.
Start ArcMap or ArcCatalog and connect to the geodatabase as the sde
user. Right-click the database connection in the Catalog tree, point
to Administration, click Administer Geodatabase, then click the Locks
tab. The Locks list opens, and you can see all locks on all objects in
Contact the user who holds the lock that needs to be released and ask
him or her finish the transaction and disconnect from the geodatabase.
If the person is gone for the day and the lock needs to be released
immediately, you can disconnect the user.
Caution: Use caution when
disconnecting users. Esri recommends that you avoid disconnecting
active sessions, especially if the connected user is in the process of
editing. You should only disconnect users to drop locks if a session
was left open even when not being used (for example, over the
weekend), if a session is orphaned and locking an object, or if a
deadlock situation occurs.
If necessary, right-click the lock and click Disconnect User. The sde
user must be a super user to disconnect users.