I assume you have been setup GNS3 and want to register X-Lite with CME (not CUCM). If yes, please check the following;
http://www.netcraftsmen.com/sip-endpoints-in-cisco-communications-manager-call-manager-express-x-lite/
Another example of SPA phones however configuration step in CME shall remain same;
http://www.cisco.com/c/dam/en/us/products/collateral/unified-communications/unified-communications-manager-express/spa_polycom_application.pdf