So the plugin is activated but you see no buttons…
Being able to use PM is a permission so you will need to enable it for the users you wish to be able to make use of it. So the first stop is manage Permissions and turn it on for those permission sets that are appropriate.
Individual users who DO have permission to use PMs also have the ability to opt out of it within their SP profile options.
You as a forum admin should see the buttons all of the time although – and I would need to check the code – it might be that if nobody other than admins has permission to use it then the buttons wont display – which would make sense…