Your amp prefers a 4 ohm load, so keep the cab switch on mono, and plug into the jack labeled 4 ohms (the one on the left when facing the back of the cab). The amp can handle loads higher than 4 ohms without damage because it's a solid state amp not tube, but 4 ohms is the only way to get the full 100 watt potential from the amp. The guy in the music store was right, and your guitarist friend probably plays tube amps, and most tube amps are either 8 or 16 ohms. Your amp is solid state, not tubes...different animal, with a different sound, and different needs. Use the 4 ohm cab input.
Here's a link to the manual for your amp:
http://www.marshallamps.com/download...0hbk%20Eng.pdf
Read page 4.
I'm not sure what's causing the squeal at higher volumes, can you try a different guitar?
Maybe your pickups are not wax potted and are causing the squealing.
Try turning the gain down a little, does that help?
Try standing further away.
Try facing away from the amp.
Does it squeal while you're playing, or just when you stop?