Before you check out any projects, you need to setup your repository:

1. Open XCode.
2. Go to SCM -> Configure SCM Repositories.
   1. Hit the little + in the bottom left corner to create a new repository.
   2. Name it whatever you like and set the proper SCM type.  I prefer Subversion.
   3. You can either fill in all the fields or type in a URL and it will parse it for you.  Here are a few example URLS:
      
      • http:// This e-mail address is being protected from spambots. You need JavaScript enabled to view it /repos/projecta
      • svn+ssh:// This e-mail address is being protected from spambots. You need JavaScript enabled to view it /home/svn/projectb
   4. Type in your password.
   5. You can test your connection by hitting Apply.  If everything is correct, it will change from "Testing Connection..." to "Authenticated.
   6. Hit OK and get back to XCode.