Whoa, whoa, calm down. Now I know you're upset, but there's no need to shout.
A clear and concise error report saying exactly what you did and what went wrong is much more helpful.
The message appears because there are certain dishonest characters out there in internetland who have websites which are set up to look as if they have lots of missions. They don't - in fact the missions belong here or other reputable OFP sites. There is a bit of a spate of it happening at the moment and the back room boys have been trying to prevent it.
I've just tried to download a mission myself and you're right, it isn't working. Looks like the protection that's being put in is so good its preventing everything. ;D Thanks for the heads up.
*heads off to the back room*