Erm, all I can really say is whack harder :p
I've had no success with prying it out - every time I've tried, It just ends up getting jammed in at an angle. Then you have to pop it back in and try again.

If the mechbox only pops out half way and hangs up on the front portion of the motor and the bottom of the stock, you should probably pop it back in and try again with a harder whack. On mine, if it doesn't come out entirely in one go, it won't come out at all.

I guess if you have some appropriately shaped pliers or something, you can try to pry it out. Or you could try popping something through that hole I was talking about (it's taped up from the factory, so you'll have to use something fairly sharp) and pulling the mechbox out with it.

Just give it a really firm whack. And I mean, REALLY firm. It should come out.

Check to see if the motor set screw is sticking out of the bottom of the motor bracket. Sometimes it will get hung up on the inside of the receiver/stock. If turn it in untill it's flush it should give you some clearance, just don't forget to adjust it back afterwards or your motor will be too tight.Untill your mech box has been removed and installed a few times, you will really have to smack it pretty hard to get it out. I usually but-smack it on my carpeted floor and it comes out after 1-2 hits.Just pretend your finnishing off a wounded foe with a but wack an he's crawling towards his rifle- that's just out of reach!

Here a trick I had with mine..

Have the gun in a slight angle, upside down (have the rail a bit under the gun...) and do the whack thing... it helps the mechbox to side out more easily.

loosen the bolts holding the whole butt together, it'll let you get a bit more flex/spread out of the gun, pull apart on the butt of the rifle, and shake a bit. It'll let it slide back, once it's started coming out, it'll basically fall out.