Adding video to template in After Effects

Start by opening the flow chart and expanding things. You can figure out which layers to trace back by soloing the layer in the main comp that has the feature you want to edit. I do this all the time because a lot of my compositing comps have nested comps that are sometimes five or six levels deep. The flow chart is the easiest way to find what you are looking for, and when you find it, a double-click will open up that comp and select that layer.

As Mr. Schultz already hinted at, most templates obfuscate their structure by hiding layers in the timeline(s) with the shy guy to prevent users from inadvertently messing up things. Turning off this stuff will usually reveal it all...

Mylenium

I would make sure that the main shy guy is turned off in the timeline. 

If you can find the placeholder in the project panel right click/reveal in composition.