Found this again after a long time. It looks like the option to learn about sewer entrances is missing since the last update! Unless I've missed something obvious, it makes it impossible to progress very far, fun as mind probing randos is.
Okay, I've had to play it myself to remember :) Follow the tutorial, you need to evolve brain probe 2 before you will receive a goal that says enter the sewers. I'm finally starting to work on the next release and hoping to get it on Steam.