Here's a list of frequently asked questions and answers:

How do I check-in my code changes?

Well, if you have commit access (ie, you're in the "Project Members" list on the home page) then you'll need to go to the "Source" tab, then click on the link that says "googlecode.com password", and there should be an option to generate a password. You then use your google ID and that password to authenticate against SVN to commit your changes.