You have to disclose it anyway if you sell your games, so it's ok.
Quote: "
14 Selling Your Games Made in FPS Creator
Under the terms of the End User License Agreement (EULA), you can sell any games you make with FPS Creator V1.0.
You must ensure that the ODE Physics Licence is abided to. This licence can be found in the folder FilesLicenceLicense-bsd.txt
You must also ensure that the copyright of FPS Creator is clearly listed in your documentation as follows:
This game was created using FPS Creator. Copyright 2005 The Game Creators Ltd. All Rights Reserved. www.fpscreator.com
....FPSCreator Manual, Page 95"
If discloure is required in the documentation, then you know referencing them is encouraged.
The loading screens between levels make a great place for these web domain ads.
A floating semi transparent hud throughout the game would be nice too.