There are three different ways that you can open TNEF files for decoding. Note that when tnefDD starts, it doesn't display a blank window - a tnefDD window will open only one you open a file.
1. Drag and drop
  1. Simply start tnefDD, and then drag and drop the file that you want to decode onto the tnefDD icon in the Dock. For example, if you receive winmail.dat file as part of an e-mail, just drag the file from your e-mail application and drop it the tnefDD icon.
2. Open from the File menu
  1. You can open a TNEF file from the "Open..." menu item under tnefDD's File menu.
3. Cntrl-Click or right click
  1. tnefDD registers itself as able to open .DAT files when you first run it. Once the registration process in complete, you can open a .DAT file by right-clicking, or cntrl-clicking the file, then selecting either "Open" or "Open With".
Once tnefDD has opened the file, it will display a list of attachments in the TNEF file. Note that not all TNEF files actually contain attachments - some contain only e-mail attributes such as font and formatting information. In such cases, the window will not contain any files.
Opening A TNEF File