Whenever I try to do so, the program does nothing. I can add powers and traits to monsters, just not to creature templates.
Edit: This is in the libraries menu of Masterplan.
In short : It's a bug, or a missing feature if you will.
You are supposed to be able to, but the software itself simply have some crossed wires in it's code, and doesn't give you any interface to apply these.
Funny enough, if you are feeling bold, you can pry open the files in a good binary editor, and force-apply traits to the creature templates. So in essence, the application can handle it, but the interface given is unable to show.
I have reported this to the developers themselves, since they are the ones that can functionally fix this.
My guess is that this will be applied eventually, but since there are many other bugs in Masterplan, they propably need to prioritize.