2 steps:
1) On Extender, on bootup it will scan for sage servers. Once it finds it, you just gotta press ok.
2) On server, install the license key that you got emailed to you. "Install License" is one of the Start.SageTV menu options if I recall.
If I remember correctly - I have to do these in opposite order, and once I install the license, I restart the Sage service (there is an Sage Service application that lets you stop/start the service with 2 mouse clicks in about 3 seconds). (FYI - I run my server in Service Mode - several benefits to that (if you aren't connecting your server to a TV directly but only through extenders)