You must first select a pack by clicking on the Select Pack Here Button. Once you've selected a pack, a list will display showing all of the compartments in that pack (on the left). As you click on a compartment, the items in that compartment will show up in the list on the right.
You can edit or add a compartment by clicking on the Insert or Change buttons (or pressing them on your keyboard).
You can change what compartment an item is in by dragging an item on the right and dropping it onto a compartment on the left. You must be able to view the compartment you are going to change the item to on the screen. You might have to maximize the screen to see all of your compartments. However, that would be a very big pack, and I suspect you wouldn't want to carry it on your back. Or, perhaps you're putting your fishing tackle box in here. That would work too.