but if I type in the name instead of $n it works.

The problem is, $n is driven through mprog_driver, via mpdamage, therefore it should have nulled on you (which it did, by saying no victim in the room). The victim is the one that sets off the trigger where mpdamage would be called...since you simply used force, there was no victim to trigger off of. set up a simple fight trigger that goes off like 50% of the time to see if mpdamage is working for you.