Pix are too small for me to see you layers.
-duplicate the video you want to key out
-on the original, add an image mask (Object>Add Image Mask)
-drag the duplicate onto the image mask (this will turn it off in the Layers tab automatically)
-in the dashboard for the image mask, choose "luminance"
-you should get your key now (text should be behind the video
-to tweak the key, add a levels filter to the duplicated video (that is turned off), and bring in the upper triangles towards the center to increase contrast - you have to play with this