Hi try this solution out.
Download this file: http://sd.stardock.com/sdactivate_legacy.zip
Please extract its contents here: C:\Program Files (x86)\Steam\steamapps\common\demigod\bin
If it asks you to overwrite the existing files say yes to all.
After you have done this it will ask you to re-register your product.
Please do so and I believe you should be able to register your product without any problems.