There is no MIB that produces this. This information is provided by the UTLite executable which runs on the Windows clients, and sends UDP packets with username, IP, and MAC information from those clients to the Campus Manager server.
However, if you are using 802.1x, and your users are authenticating using a client supplicant, the username will be available in the dot1xAuthSessionUserName object from the switch to which the user directly connects.