Problem Compositing an Explosion

First sign of a bad tutorial: It's too long. Anything longer than ten to fifteen minutes indicates that either the creator doesn't know what he's doing and is just meandering around or it's packed with too much stuff that is never properly explained and that's just what happened here. The guy can't make up his mind what he's actualyl trying to explain and just skips around. better go to the original source as already advised by Rick.

Mylenium

I'm not surprised that you are having problems. There are so many things left unsaid that it would be easy to get lost. The tutorial is nothing more than a recipe put together by a well-meaning enthusiast. The workflow is not very efficient or effective. Copying and pasting positions are not nearly as advantageous as Shift + parenting the explosion and the debris layers to the null. Using a solid instead of a null and making it a guide layer also will help you get the explosion, and debris layer pointed directly at the camera so the perspective won't do weird things. 

Do you have an EXR sequence for the Ground Blast? Did you successfully extract the matte? Are your layers in the correct order? Without a screenshot of your comp with the modified properties of the problem layers revealed, it will be hard to figure out where the problem lies. To show us the modified properties of any layer, select the layer,  press 'uu' and take a screenshot, copy the file, then paste it to the reply field on the forum or use the toolbar to insert it so we can see it. 

Make sure you vet your trainers. Most of the After Effects tutorials you find on YouTube are not much more than simplistic recipes shared by enthusiasts, and most of the workflows are often not well thought out or well explained. There is so much that has been left out of this tutorial. It is almost a complete waste of time. The number of views has nothing to do with the quality of your training.

You might want to switch tutorials. This one from Andrew Kramer gives a lot better workflow and is easier to follow. Andrew is a pro.

Let us know if you need more help.