The newer versions of MRTG allow you to use port names. I recommend using this over the trying to define the OID instance. This has two advantages: it is easier to see in the MRTG.CFG file, and if the OID instance changes (which I've seen this happen...

I had the exact same issue. You need a "T" train of IOS. Look for 12.2(15)T1 for your router. Using the Software Finder on Cisco's site, look for the feature called NBAR Real-time Transport Protocol Payload Classification. Without this it won't class...