Hi.
1 Draw on the video layer - it needs to be wide enough to be able to see a small bit of the green bars through (it's not critical how wide it is but, if at the end, the particles are starting too far away from the top of the keyboard you can always make it thinner).
2 To get Precomp 2 into the main comp select it in the Project Panel and drag it into the main comp
3 Start off the screen draw a line exactly along the top of the keyboard and off the other end of the screen, click and continue the line at a 90 degree angle to below the bottom of the screen, click and continue the line below the screen to the other end, click and making a 90 degree turn join the line to your first point. The only critical thing is the line running along the top of the keyboard, the rest can be really rough (not 90 degrees) as long as it is drawn in the area off the screen.
I trust you have read this whole thread as the original poster had some issues which you might have as well.
Good luck with it.