I have had a few reports of people having trouble running Mediocre using Arena. The common theme seems to be Mediocre playing the opening moves and then stops.
Arena has a local opening book feature that it uses for both engines playing. Mediocre currently does not support this! I have not looked into it yet but my guess is Arena sends a 'force' command and then plays the moves from the opening book, and when it is done hands over control to the engine again.
Since Mediocre v0.11b does not support the 'force' command it will have no idea what to do after the opening.
That is atleast one thing I can think off, perhaps there is a problem with setboard as well. Anyways I will look into to it and hopefully I can make it work.
For now you will have to turn off the Arena book in order for Mediocre to play.