Hi Andrew,
You should definitely be able to get into design mode as user "admin" (not just any user with admin privileges!). Unless you have changed the settings or deleted your userdb folder in Mango/web/modules/dglux/web/
One way to alleviate this would be to reinstall the DGLux module after deleting the existing module folder. I am guessing you do not have dashboards yet?
Alternatively, you could...
Move your Mango/web/modules/dglux directory up a level (so that it doesn't load), Download it again, Open up DGLux, Log in. Shut mango down. Displace the userdb directory in your original DGLux module, and move the userdb directory from the new DGLux directory to the old. Move the old DGLux directory back into the Mango/web/modules directory (overwriting the new one).