I have added both Graham and John as project owners, so you should now have the same privileges as I have.
To commit to the SVN repository, you need to tell SVN your google.com password. If that does not work and I can help somehow, let me know.
I added a folder "R" in the trunk, and below that a folder "SDE". Graham, you could create another subfolder for your specific model ("2dpredatorprey", for example), and check in your script there.
SVN lets us change the folder structure later, without loosing any information about the history of the files, so we can always rearrange it later.
@John: Let someone who knows SVN already install a client on your machine and show you how you
a) create folders
b) check in new files,
c) update existing files,
d) compare versions and
e) roll back a change.
That's all you need to know. You already know much more about handling folders using the windows exploder.