Create a mask....make sure the layer is selected, then select one of the mask tools (The rectangle is the right one for this job), now add either a mask at the bottom to cut out what you don't want and set the mask to substract in the timeline, or add a mask for what you want to see and makse sure the mask is set to add in the timeline. You can also create a new square solid and use it as a track matte for your image. You really do not need a filter to mask.
Dustin - if you are new to the Mask tool in AE, you'll want to read help on how to adjust it. It may not appear as friendly as what you're used to with FCP, but it works well. I use both methods mentioned pretty frequently.