I can't find The Reef in House of Wolves. I bought the expansion, but every time i click on it, it just plays the trailer. Am I supposed to go on The Tower and get to it there or something?
If you're just clicking the icon near The Tower, then that's just for the trailer.
When you progressed through the original story missions for Destiny, it took you to The Reef several times. It is located at the top of your map, between Venus and Mars. From there, you should have access to visit The Reef.
Of course, there are requirements to begin the first mission. You need to have completed the prior story missions and be at least level 24, though level 26 is needed to complete all of the story missions, and level 28 is recommended.