When you first call up the transwarp menu, "Sherman System" is the option for "transwarp to Station K7". You won't see K7 until you open up the Sherman System option.

Also, double check your "in progress mission log", and make sure everything related to the CXP rank up is complete, both the store mission and the talk to get the rank in the first place.

On a related note, do you have the option to pick "consul" as a player title and does it say consul for diplomatic rank on your status screen?