I'm looking for the "Rannoch: Admiral Koris" mission. Who or what do I need to interact with to receive this mission? I know it's possible to permanently miss certain missions by not accessing them before completing other missions. Is this the case with "Rannoch: Admiral Koris", or am I just not wandering around in the right places?
You'll get this mission at the same time you get Rannoch: Geth Fighters. If you haven't advanced the plot to this point, you won't have this mission. This happens after you've assaulted the Geth Dreadnought.
Note that it is important to play the Admiral Koris mission, and the other side mission on Rannoch (Geth Fighters) before completing Priority: Rannoch, as it's possible that the outcome of Priority: Rannoch will be influenced by whether or not you've completed these two missions. Completing either of these two missions will unlock access to Priority: Rannoch.
You get this from the admiral in the War Room after the Geth Dreadnaught mission.