Launch the application from your Applications folder or your dock if you've placed it in there..
Select File > Open… and find the winmail.dat file on your computer.
If the file successfully opens, you'll be presented with a window showing the contents of the file.
Note: Your file may show more or less information including the body of the email.
The files can be exported from the open winmail.dat file in a number of ways:
Note: There is special handling of calendar invitations and contact/address entries sent from Exchange users. Each will appear as a file that you can export from the winmail.dat file: