The steps to install tnefDD are:

Download tnefDD
The latest version of tnefDD can be downloaded from tnefDD at SourceForge.net

Unzip tnefDD.
tnefDD comes in the form of a ZIP file. To unzip the file, simply double-click on it.

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