It's not ideal to leave modeling chocolate in the fridge but it sounds like you've got to do it! You can avoid the condensation issue (moisture makes modeling chocolate a little wet and sticky) if you keep it well-sealed in plastic wrap until it's ready to use. Then the condensation forms on the plastic wrap instead of on the chocolate's surface. Just to warn you, modeling chocolate is harder to work with when it's cold so you'll want to let it warm up to about 60 degrees F before attempting to use it.
Here's some info on how to work with rice treats and modeling chocolate: http://www.wickedgoodies.net/2010/08/cake-sculpting-with-rice-treats-and-pvc/