There's a long thread about the May TNT here:
You don't mention the version of NED you are running. Is it the latest version? You may need to follow these steps to download the May TNT2:
1. Launch NED and select the TNT2 tutorial in the Library and click on the delete button.
2. Quit NED. You can empty the trash if you like
4. Open a Finder window and go to your user account > Library > Preferences
5. Find the "com.nonlineareducating.NED3.plist" and the "NED_Player.plist" files and delete them. (Don't worry if you don't find the NED_Player.plist files, but it's important to delete if you find it).
6. Open NED
7. Click on Downloads & Admin and make sure TNT2 is enabled in the Tutorial list and click on 'Download'
Hope this helps & enjoy the May TNT!!!