Try the following:
1. Make sure NED is closed.
2. Delete all version of NED on your computer (NED 1, 2 and 3).
3. Open a Finder window and go to your user account > Library > Preferences
4. 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).
5. Download NED 3 again here : http://www.macprovideo.com/meetned/
6. Start NED
7. Enter your account info when prompted.
8. If you changed the Download Folder location, you'll have to do that again under NED > Preferences.
9. Click on "Downloads & Admin" and press the "Download" button.
If you are still not able to download the April and May editions please do the following:
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
3.Â Repeat steps 1, 3 and 4 above. (No need to delete NED!)
4. Open NED
5. 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!!!