Christiaan,
After a quick look at your link I'm guessing you are talking about the post with the 45 degree angle brace. In Ac11 (not sure about other versions) you could use the Strut12_11. It's in the Roof Maker 11 library. It also allows you to change the angle. You should add your AC version as well as computer specs to make it easier for others on the forum to assist you.
Doug