The mission downloads are not offered for legal download anymore - although you can still d/l the starter pack. Copyright laws sometimes work that way.
The only legal way to get it is to purchase a copy of Prophecy Gold (which is really worh it) - I'd advise ebay.
BTW: This may not be the best forum to post these questions, as the guys here talk about the Privateer Remake. Go to http://www.crius.net/zone/index.php? , register for a free account, and have fun browsing the forums - a lot of questions you still may have are already answered there.