I just downloaded the map pack and the first thing I did was check out the new stamps in the editor.

I noticed they didn't show up. So I went to the install directory and I saw that there were now two stamps folders. One /Core Stamps/ and one /CoreStamps/ (the latter being the new one).

I moved the stamps from the latter to the former and they then showed up in the editor.

I moved the stamps back to CoreStamps then removed all old stamps from Core Stamps. I restarted the game. If the game was reading the DLC stamps from CoreStamps, they should only pick those stamps for new maps. But instead of only picking those stamps, the game picked no stamps - as if it had none to choose from.

Conclusion: You installed the new stamps to the wrong folder, and the game can't read them.