When you open Mediasite Desktop recorder you will have the following screen, It says in the top left you need to Register to Sign In.
To do this you will need to go to MyMediasite and sign in with your username and scu password.
Once you get the following screen click Download the Desktop Recorder.
You don't need to download and install the recorder as you already have it on your computer. So please click the button Register Mediasite Desktop Recorder.
You will get the following pop-ups, your Mediasite desktop recorder is now set-up correctly