You can open winmail.dat files in a number of ways:

  1. In most apps, simply touch on the winmail.dat file. WinDat Opener will automatically be used to open it. If WinDat Opener doesn't automatically open, then it's probably that another app is set to automatically open any ".dat" file. In that case, either delete the other app from your device, or use one of the other ways to open the file.
  2. Touch and hold on the winmail.dat file. You will see a list of options; if WinDat Opener isn't listed as a main option, it will be a sub-option, usually of the "Share" main option.
  3. Use WinDat Opener's file manager screen to find the file that you want to open, and touch on it. Note that the file will only be visible if the location containing the file is file manager enabled. 
  4. Drag-and-drop the winmail.dat file onto WinDat Opener. Note that not all devices support drag-and-drop.