I just got some CK gumpaste mix, you just add water. Should I store the left overs in the fridge until I need them?

Doesn't it say on the box? The Witon one says that it can be stored in the fridge for up to 6 weeks, or outside for so many weeks. I forgot.

If not, do they have a website where you could contact someone?

http://www.ladycakes.com/Recipes/Frostings/GumPaste%20.htm

This is a good article about gumpaste .. hope this helps!