I can point you in the general direction of what to do from Winbox. I can't give you straight terminal steps.
Click "PPP" along the left.
Click "PPPoE Server"
Add a new PPPoE server on the appropriate interface
Click the "Secrets" tab on the top
Ensure that "Use RADIUS" is checked. If you want interim updates, put that in as well.
Click "Radius" along the left.
Add a new RADIUS server. You'll need the IP of the server, and the RADIUS secret. You'll also need to configure the RADIUS server to accept connections from the Mikrotik.
Things should work now. Look at the logs for info if things aren't working.