Here is a great tutorial "How to use TortoiseSVN to access Mediatemple subversion over svn+ssh", however everything worked for me apart from gaining access via TortoiseSVN. Every time I tried to connect I would get this error message: "Network connection closed unexpectedly."

Eventually I discovered that the PuTTY Configuration 'Default Settings' must not have any user entered data stored in it. The only way I could get this to work is to create a new Saved Sessions value and store my data in there. Also rather than putting my username in with the address (as done in the example above), I entered my username into Connection -> Data -> Auto-login username.

i.e. Settings for new Saved Session:

Session -> Host Name (or IP address) : s#####.gridserver.com (##### your gs number)
Connection -> Data -> Auto-login username : serveradmin@yourdomain.com

Then using PuTTY + Pageant I was able to connect using TortoiseSVN.