You must follow three steps to install tnefDD.
 
  1. 1.Download tnefDD. The latest version of tnefDD can be downloaded from tnefDD at SourceForge.net
  2. 2.Unzip tnefDD. tnefDD comes in the form of a ZIP file. To unzip the file, simply double-click on it.
  3. 3.Drag tnefDD to the Applications folder. Drag the tnefDD.app file to the Applications folder and drop it there. If you prefer, to you can also drag it to Applications/Utilities
 
Once you have followed these steps, tnefDD is installed. Note however that it may not register itself as a handler of DAT files until you have run it for the first time.
 
Installing tnefDD