As this is a .vap file, there is nothing to install into DCS itself; You need to import the profile into VoiceAttack.
Click the
button on the main window, choose "Import Profile", and browse to where you saved the downloaded .vap file.
The location of that file is not important (as long as you can find it), as the file is only used while importing its data into VoiceAttack's database. Afterward the .vap file serves no purpose, other than to restore the profile if you make a mistake editing the imported copy.
For more information on VoiceAttack's features, press F1 while a VoiceAttack window has focus, and VoiceAttackHelp.pdf should automatically open in your default PDF reader.