Well, I would assume that write
-access to the Avidemux SVN repository is restricted to the developers for good reason.
Just imagine some \"anonymous\" hacker uploads malicious code to SVN and hundreds of users check it out
So in your case you must ask one of the developers to review your code and, if accepted, commit it to the SVN for you.
In case you are actively joining this project, the devs could create a SVN account for you...