A nail will not work as it will not be in the center of the whole. Probably a trip to the hobby store is in order for a balencer. They come in a variety of configurations depeding on your budget. Once you get the balancer then you can use Geistware's method to get the prop balanced.

After I got past training where I didn't break props on a regular basis, I switched to APC props because I hated balancing MA and the like. But for training, the MA job you have is perfect and it's pretty tough.