Once you have registered an account you must enable "Add any gadget by URL". To do this, go to the gmail settings, then to labs, and scroll all the way to the bottom where this gadget is. Enable it and click save. Once gmail has reopened there will be a new menu item at the top next to Labs in the settings. Here you can then enter the following URL:

http://gwatch.info/app/gWatch1.0.xml

Enter this into the box and it will then show up right below chat and then you can log in and start tracking your time!