A few times a year I get asked how to bike into and out of the airport in Milwaukee. I have put together a Google Map (see below) to show the route I prefer. It uses a side service road and a pedestrian crosswalk to avoid all the loops that everyone in motor vehicles are using. The first trick is to get off Hwy 38 (S Howell Ave) and onto the service road also confusingly called S Howell Service Road. You can see this access drive at 4918 S Howell on the Google Map I created. It is marked by the green bicycle symbol. You can look at it on this page below and pan around to see the entire route, or you can open it up as a larger map on another page to get a bigger view.
Follow this service road through the parking lots until you get to E. Joseph M Husteiner Dr. and then turn left, or east, to get into the airport. Ride on Husteiner past the international terminal all the way to the end of the road where you will see a crosswalk that takes you over the median to the baggage terminal. Here you can cross the drop off/pick up road and park you bike under the overhang of the parking structure. There are bike racks here next to the motorcycle parking at the NE corner of the parking structure. I have left my bike and Burley trailer locked up there for over a week and it was fine when I got back from my trip.
If you use Google Streetview to look, you can see the bike racks under the overhang.
Good luck and Bon Voyage!