yup one of those mental blindness things I think. Even after you said I could not find it and then did a control f and low and behold its like right there. don't know how I did not see it. even has a mail icon.
Do note that you can't currently message users on other instances on kbin. The button will still be there, but it will bring you back to the homepage. I believe Ernest said something about it being "unimplemented," so I'm assuming cross-instance messaging will happen someday, but is probably not a high priority at the moment.