I have just bought this plugin and have discovered issues with the clearly broken and untested code mentioned above.
I'll have to undo what I have set up so I can start from scratch, but the better outcome would be that someone from wpForo looks at this thread, sees the obviously broken and untested code and fixes it. From what people have said above, it doesn't look that complicated, but it takes someone to want to unit test the member search page.
It's a shame because the free plugin works so well, it has inspired me to invest in an addon which has not been unit tested.
I'll be back later with my first bug report of what is going to be a list of bugs unless someone looks at this thread.