What erischild said, plus you do have to research various techs to unlock different missions. These are spread throughout the tech tree--the blue symbol on the tech tree is used for these (look at Colonial Settlements, for example, third blue icon out of five: "Research Mission"). Hope that helps.

Thanks for the replies, they have been a help. So I know now how to look through the tree for missions, although I wish it was searchable. Also I'll read the fine print of what a new Government type does or does not allow!