Unless I am missing something very obvious, I don't see how I can branch on CodePlex using SVN. I'm looking at the popular projects and can't see anything about a trunk or branches.
What am I missing? Also, I'm using TortoiseSVN.
SO folks have shed some light on this here: How to use an Internet Subversion respository when developing code? - you might want to take a look and revisit how you want to go about your task. Hope this helps.