Yeah I remember having issues finding this at first as well, because the utility starts basically in the options menu. Click on the house button in the top left corner then you see the top line gains 2 drop down menus. Choose your character in the left one and Gear Buff Macros in the right one to be able to use this to its full extend. Some items have more than 1 ID and you'll need to find the one that corresponds with the item you have.
Carpe noctem! ... Get a life? Well, once I nearly found one, but the link was broken
Disclaimer: If you find spelling mistakes, be proud, grab a cookie and then spent a thought about me having german as my mothertongue