If I understand correctly, you want to use your text layer as a matte for the video layer (ie, display the video content inside the text), right?
If so, there are a couple of ways to do that.
One is track mattes.
Make sure the text layer is located directly above the video layer in the timeline.
Then press the "Toggle Switches/Modes" button at the bottom of the timeline panel.
When you do this, AE will display in that column the blend and track matte modes instead of the layer switches (motion blur, adjustment, 3D, etc). They will return if you press the button again.
Select the video layer, and in the TrkMat menu select "Alpha Matte - (your text layer's name)".
This means that the transparency information in your text layer will be used to mask the video layer.
Of course, this works with any layer type, as long as the masking layer is located directly above the masked layer in the timeline (a way to bypass this requirement is to use the Set Matte effect instead of the Track Matte controls, but we don't need to get that fancy just yet )
For more information, see Track mattes and traveling mattes in After Effects Help.
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If I understand you correctly, you would want to place your text layer above your video layer in the timeline, and select the "Alpha Matte" track matte on your video layer (this will force your vide to use the alpha information of your text layer).
edit: LOL. Adolfo, you beat me by a hair.