Cannot type in the cost field of the assign resources dialogue box

[cldonley]

Well, the title pretty much says it all. I'm working through MS Project Step by Step guide, in chapter 4, and I'm trying to assign a cost to a resource in the assign resources dialogue box. It's not working. I verified the problem by opening an existing project and attempting the same thing, no joy. Is there another way to add a one-time type of cost to a resource, such as travel? Thanks for the help!