There are four steps you need to follow when setting up a quantity price break:

1. Check you have allowed multiple prices in your site settings > Use multiple prices on products?

2. Edit Product [Admin: Products Manager -----> Edit the Applicable Product]

3. Edit the Prices & Delivery Tab [Admin: Edit the Prices & Delivery Tab]

Tip: For full details read section 5 of this article in the REC Guide. 

4. In the Prices Table, enter the relevant quantity ( apply a user group if applicable, otherwise leave blank to apply to all users of the site ), then enter the discounted price break, and update. Add any further price break`s if you need too.

NOTE: Remember to TEST what you have added, by running through a demo transaction up to the checkout screen, to make sure the correct prices are being applied.