Thank you for replying, I'm running into problems with this for days with no progress. Anyway, the snippet doesn't give me any errors but isn't working either so far. It's returning null at the moment, I believe due to the where clause not working.
DataSet ds = tree.SelectNodes(CMSContext.CurrentSiteName, "/%", "en-us", false, "CMS.MenuItem", "MenuItemGroup = 'Upper Top'");