Access the Tier Pricing List page via the Tier Price List in the left hand menu.
You can the press the "Action" button on the list you wish to add, then thece Add Products into Tier.
Products can be found by navigating through the category's or by searching for products.
You can then insert the price you want for that product, then select the the Tier you are adding to, and press the Add button.
The product will now be added in to the tier.
You can click on the price to quickly edit it.
To view all products in a tier, select view or edit products in tier from the action menu on previous page