After downloaded the license file I can't seem to open it.

You are not supposed to open the license file directly. Instead please click at the browse button on the register window (where you find the 20 digit machine ID), navigate to and select the license file, then click "OK" to confirm. That's how you use the license.

More details with pictures can be found at the online guide http://www.dvdae.com/dvdae/tutorial/license.