ZC does this wonderfully, you just have to know where to look.
First, set up the product you want to have quantity discounts on in Admin/Catalog/Categories Products (where you normally list your products). Enter the price for ONE item as the price.
Then, click on the

for the product you want in the list of products.
It should take you directly to that product's 'Price Manager', but make sure the right product is shown above the "Edit" button or you'll wind up making the changes on the wrong product. (Don't ask me how I know that.) Under the red "
Preview Only ... Current Price Status" is an "Edit" button; click on it.
Under a lot of filled-in boxes you will see a button labelled "
add blank discounts."
The number of blank discounts to be added at a time can be set in the config settings, I don't remember exactly where but it doesn't matter, if you want more blank boxes than the number given, just click the button again. Five is probably the most you will need anyway.
Then fill in the first quantity at which you will offer a discount, and the amount or percentage. The "Discount Type" box will tell ZC how to handle it, and you can discount either from the official price or a special price you have on the product.
Repeat until you have filled in all the quantity breaks you want to offer. I suggest discounting an amount or a percentage rather than to a given price, because that way, when you raise the base price, the quantity break prices will adjust right along and you won't have to edit each quantity break manually.
Be SURE to hit "Update" to record the quantity breaks you just figured and typed in!That's it. Just check your product "cart side" (what the buyers see) to make sure everything looks right.
If you need to make any changes to price, just repeat the above steps.