Hello! I have made a couple cakes with chocolate ganache as the filling in a couple of cakes, and instead of it being creamy when I cut the cake, the chocolate has set up. It's not hard but not creamy like frosting. Is this the way its supposed to be or am I doing something wrong?

Yep, that's what it's like!

If you want a creamier, frosting like ganache you can whip it. I love a layer of whipped ganache topped with a layer of poured ganache. This is the recipe from CC.
http://cakecentral.com/recipes/whipped-chocolate-ganache/

If I want a creamy ganache I use less chocolate and do a 55%-60% whipping cream and 45-40% chocolate. Also I find it depends on what chocolate I am using.... chocolate chips, Candy wafers or true chocolate. I believe the chocolate chips and candy wafers ( like Merkens and Wilton's) have a fat product in them to stiffen up. I also do whip my ganache. Play with the percentage of whipping cream to chocolate depending on what chocolate you are using. See if that helps.