PS CS2 help needed!

Hey guys, this is my first time asking for help here because, usually, with enough messing around and experimentation, I can solve the problems I'm having, however, this time, I've had no such luck. :'(
I am making a banner and I desperately want to do rounded corners...I cannot figure it out AT ALL.
I even tried following 2 different tutorials already with no luck.
If anyone can help me I would appreciate it soo much!!