I've entered a misspelt display name upon registration and now I can't find a place to change it to my user name... My Stuff doesn't have any controls to do that.

That is only available for changing if the admin has specified the ability to do so. If you're refering to this forum it's turned off, but I can change it for you if you'd like.

But why does one need to specify one in the first place? It should probably be an optional step...

Yes, change it please to match my user name, thanks