Can you seal a plaster mold with poliurithane?  

You could.  Not usually do people use that, but you could use it for a temporary application.  Only problem with using a polyurethane, is the fact that you will have to keep applying it after every 5 coats or so, since polyurethane is a plastic base, and there for, will melt along with the plastics you pour...  

What do you suggest that I use?  I have a mold and I need to know how to seal it.

I've tried lots of different things to seal them with, but the best and easiest in my opinion is Elmers Glue All.  Just mix it with water 50/50 ratio and flood the cavities.  I will give it 2-3 coats and it produces a very shiny bait and I haven't had to reseal any molds yet.