Proton is stored on a Subversion server, this helps you easily see what has changed from day to day and makes updating a breeze.
First install a svn client such as TortoiseSVN.
Next, SVN “check out” the following URL:
It will download around five megs of source code and media, including the entire engine and some examples.
If you'd like to contribute something to the project or wiki please do! To make changes on the wiki you just need to make an account. Be bold!
For code/script/example fixes/submissions the best way is to post a svn patch (or just a source code snippet) on the Proton SDK forums.