I haven't had good luck with Mod Podge either. I found it was messy to use, and took a long time to dry. Also, I had the same problem with warping. Not much helpful advice, I know, but I do feel your pain.
All the stuff I've read about Mod Podge says that since it is a water based adhesive, you must apply really thin coats to avoid saturating the paper and causing warping.
Since you've already finished your project, that doesn't help much.
As a last resort, I would probably try lightly misting the page with water, and then drying it again under something flat and heavy.
I make lots of altered composition books, and have been applying the cardstock to the front with my ATG gun. It works well, holds up with use, and needs no time to dry. You also don't have bubbles to worry about.
Good luck and hope you can salvage your project.
Help with Warping Pages?
