For anyone who's experiencing permissions problems with this mod (as I did for a while), remember that there are two sets of permissions that need to be enabled.
The main one has been well covered here and is a Usergroup option:
Usergroups >> Usergroup Manager >> Edit Usergroup >> Post/Thread Permissions >> Can Mark Threads as Solved and Unsolved.
The second and less obvious one is via in your thread prefix manager:
Thread Prefixes >> Thread Prefix Manager >> Solved Thread >> [SOLVED] >> Edit Permissions
Hope that helps anyone who missed the second one as I did.