The other option would be go to camelotherald.com. Seltect downloads, and download catacombs free trial.
After going through this rather lengthly installation, when you log into the game, you can purchase the two expansions (Darkness Rising and LoTM) from the in-game billing section. It is 29.99 to buy them both this way though, and you can almost definitely find the 5th anniversary edition cheaper than this, and usually you get a free month anyways when you buy the boxed sets. If you are in USA, do an amazon.com or some other search and you will find it. If not in US, it can be trickier to find I hear, but I know others have found it as well.