There are various countries that are blocked from downloading or posting anything on Sourceforge.net due to USA laws. If you're in one of those blocked countries you won't be able to download from Sourceforge.net. But it may also be due to STK project settings, the export control option will limit the download's availability. Also, as stated int SF's TOS:
Users residing in countries on the United States Office of Foreign Assets Control sanction list, including Cuba, Iran, North Korea, Sudan and Syria, may not post Content to, or access Content available through, SourceForge.net. Geeknet handles user Registration Data in accordance with the Geeknet Sites' Privacy Policy
In short, it seems that you are being blocked due to your IP address belonging to one of the blocked countries, it may be an error, but I would advise that you post a support request in SF to confirm.
Posted: 03 Jan 2012, 06:57