Um, you don't open a track matte. You use a track matte -- a piece of footage in your project window -- to affect the transparency of another layer. It does by using its own transparency or by its luminance. Very handy things, them track mattes!
Start here to learn all about 'em:
Press F4 to switch the columns in the Time Line until you reveal Track Matte, put your matte source layer above the layer you want to mask with the track matte and then select Alpha, Alpha Inverted or Luma or Luma Inverted on the layer below your matte layer. If that doesn't work, or even if it does, go through some basic AE training and learn to use the Search Help field at the top right corner of AE. It's will give you answers to almost any question you will ever come up with.