FYI, there is little to do in Concourse D.
Concourse D is "T" shaped and there is a gift shop, Starbucks, Wolfgang Puck's Express, Bar, & Pizza/Bagle stand at the "T" intersection.
If your layover is longer, leave the secure area and visit Renaissance Books and the Billy Mitchell Cafe in the Main terminal (no more than a 7 minute walk from anywhere in Concourse D.
Check out the guide to the Milwaukee airport link below for more info.