First make sure that it is enabled by going to Catalog Set Up and set the "Price Discount" field to YES. Then click Update Variables
Now, when you're on the product screen you'll see new fields to enter the quantity break and discount amount. The discount amount can be either flat $ amount or a % off

ex.) A store wishes to offer a discount of $1.00 off the total price when 2 items are purchased, $2.25 off the total price when 3 to 6 items are purchased, and 10% off the total price when 7 or more are purchased. They will use the following settings:
2 or more - 1 ($ off)
3 or more - 2.25 ($ off)
7 or more - 10 (% off)
The discount breaks will be listed on the product page to alert your customers that they can get a discount for purchasing more then 1
