I'm guessing you're in Full Screen View. Press ESCAPE to come out of it then you will be able to type in the description.
Same applies to Chat during a game - you can't write in the chat box in Full Screen View. There was one other place you can't do in FSV, but I can't for the life of me remember what it is now!
WGT - why can't we write in Full Screen View? Is this something that can be fixed? I always play in FSV and have to constantly keep switching views to be able to type. Until it is fixed (if it can be), it might be an idea to put the above into the Troubleshooting section in FAQ's.