Emtaym- If you go on Doax's DIY on installing the underdrive pully, there is a picture of it where he prys it with a crow-bar to put the belt back on. that picture will give you an idea of where it is, you cant see it looking at the engine from the top.

I personally spent the extra bucks to have my mechanic install a new tensioner and water pump. that ensures that not only the mechanic installs the belt, but also has to make sure the tension is correct since he put on a new tensioner. I beleive when a mechanic says something is not needed, he/she is just too lazy. Another part to put on is just more money in their pocket, and if they say no; then something is up. thats just my take on it, unless they are a friend and they are honestly trying to save you money. dont second guess yourself, if you feel it is needed, do it.