Brandon -- I checked out the kit. It looks like it's been around awhile. the Stewmac plan won't fit the Martin kit exactly, but it will be close enough.
The kit looks okay. Don't do anything extreme to it like "heat" or "water": heat improperly applied would dry and crack it and water would make it want to relax more, worsening the problem. Humidifying and molding slowly will do it.
1) Stabilize the humidity of the wood parts by storing them in a room of from 40 to 50% Relative humidity for at least two weeks. Like all our moms used to say, "You don't know where it's been!" If the seller's "friend" is from Seattle or from Phoenix, it makes a difference.
2) Go to the hobby store and buy some foamcore.Cut out the shape of the guitar from the full size Stewmac plan and fit the sides into it. Do this BEFORE you start building your external mold, and put your foamcore and sides in the humidifying room. In two weeks or so, they will be close enough to the original shape that you can put them in your external mold and not have a problem.
Did the Martin "Manual" come with the kit? If not, you're not missing much. Just forget it ever existed and your kit will turn out better. Go to www.stewmac.com
and download from "free information" their manual on their dreadnought kit; it is close enough to the dread kit you have and much better than the sketchy manual from Martin.
If you want some good detailed instructions, go to www.KitGuitarBuilder.com
and follow the steps in the "Martin Instructions" pages -- almost showing the complete kit. (It's a jumbo, but that has no bearing on the process.) The instructions shown follow the Martin manual and make up for its shortcomings.