http://www.fpscreator.com/license.html
Quote: "Selling games you've made
We are pleased to announce that there are NO license restrictions on FPS Creator. You can sell whatever you make and do not have to pay us any royalties. This entitles you to sell your games on your own web site, sell via a publisher, have them placed on magazine cover discs or just give them away for free. The choice is yours. While we'd love to hear about any success stories resulting from FPS Creator built games, you'll never have to buy a license from us."
Im not sure if this applies to the model packs aswell though