Advance Mart is a progam that lets you change what the PokeMarts sell and in What town. Although it does not let you change What items work how or their names. This tool is for changing the marts inventory and how much it's priced.
So if you were only looking to hack to cheat your way through the game, you could use this for making the first town with a PokeMart sell Masterballs for 100 whatever the currency is.
So, in my opinion, unless your making a ENTIRE new game with Pokemon Gameplay (example Mileseeveeprower and her Pokechao game) this is kinda- well not USELESS but it does have it's weak and strong points.
First you select: File->Open ROM like all programs. Browse through your computer in the little window that pops up untill you reach where you have stored your ROM.
To select a PokeMart you either select it from the world map on the top or the slider on the side.
Then to change an item you click on a Pokeball and then on the lower slider you select an item and that is what the pokemart will sell.
To change the price ALL PokeMarts around the reigon will sell an item for you to to Tools->Item Editor. This function lets you change the price an item is sold for.
This video is by the hacker Tamashii Hiroka. used with permission
Um, for some reason the videos don't work so I'm providing a link.