Sorry but I am completely new to all biking at this level.
Bought a new Mongoose and if I put any pressure on the pedals the chain pops and slips...I guess it slips. Is there any adjustment I can make to rear assembly to correct this before I take it back?
If you don't know what you are doing you will probably make the situation worse. Random adjustments are a very poor approach.
Take the bike back to the dealer and have them adjust it properly. Better yet, have the mechanic show you what he is doing so you are better informed next time.