eLicense Error 501 is one of the most common errors encountered. It usually indicates one of these three problems:
1) you have typed in your key incorrectly. This can be a simple typo, or an incorrectly copied&pasted key, or - if you're using Offline Activation - you may have copied only part of the license request.txt file.
2) you are using the correct key, but for the wrong game. Make sure that they key you're using is valid for the game you want to activate. Please note that if you own CMSF and a module, you need to use the MODULE key! It automatically activates the base game. If you have a module and try to use the base game key, you're going to get a 501 error as well. Please check out the Combat Mission Knowledgebase for more details about modules.
3) VERY RARELY, it is possible that the key is valid and you're using it for the right game, but that it has not been registered correctly on the eLicense servers. After you checked 1 and 2 above, please email firstname.lastname@example.org with the subject line "Error 501", and include the game title and the key you're using. Your key will be doublechecked usually within 24-48 hours (and often much quicker than that).