Oy, Cadillac. The Cadillac has two stage doors - one internal to the hotel, the other out to the street. The vast majority of the cast used the outside door when the tour stopped there in 2005, the first time they used that theatre (previous stops were at the Auditorium). The outside door is really obvious - if I remember rightly after this many years, you go past the hotel doors to get around the corner, then it's in the rear side behind the Dress Barn (if the Dress Barn is no longer there, the retail space definitely will be). There's a dumpster or two back there and the loading dock.
If people are staying at the hotel, they may use the internal doors, and I have no idea how those work (the leads for The Producers were using those during the out of town tryout - I was more into fangirling the ensemble so I was fine with not seeing Nathan and Matthew), but the Hotel Allegro is not cheap and people who are staying elsewhere (which is likely to be the whole cast) will use the loading dock door.
Unfortunately, I can't help with Des Moines. I hate their civic center with a passion, but I never stage doored it. Who builds a barn of a theatre without balconies? Des Moines, that's who. Awful seats for Miss Saigon back in 1998 and I never went back. The place is huge and made of concrete, as civic centers of a certain age are, and a walk around the building will definitely show you the stage door. The tour continues to tack up their Cosette signs for the benefit of the actors, and Cosette is a fan's best friend. The best bet is to see what part of the building the theatre is in, then look at the opposite side of that section compared to the main entrance. Alternately, with a Midwestern venue like this, look for any part of the building with staff or handicapped parking adjacent to the building - sometimes this will work better for you than a loading dock (Tampa and Peoria have stage doors separate from the loading dock; lots of theatres have it on the loading dock).
What kind of literature and what kind of life is the same question. - Tom Stoppard