You have to put your left hand flat on the top of your head while you rub your stomach counter-clockwise with your right. Don't forget to hold the steering wheel with your left knee and the bus will shift itself. Congrats on the new bus!
Start in 1st low. When you're ready to shift to high range just pull the switch and let off the accelerator whilke it shifts. You MIGHT have to declutch if you're in a minding situation such as a steep uphil or downhill run. When you're rady for second just declutch and push the switch while you shift to second. Keep repeating all the way up. There will probably be some instructions in the bus somewhere printed on proper shifting technique. I have a feeling you won't find you need that two speed rear end all that often though.
If you would have changed the rearend in the other bus, you would have had more than the 2500 price of the new bus invested and you still wouldn't have had the exhaust brake. I wish I had an exhaust brake coming down the Grapevine. I have not looked into this yet, but several people have told me that a DT466 is dirt cheap to rebuild, like $1000-1500 for a rebuild kit. That doesn't mean that you want to rebuild the engine as soon as you buy the bus. I would ask them if you could have a mechanic look it over. Even if that cost $100-150, that would be some of the cheapest and smartest money spent. Good luck.