There are two ways you can add more products:
1. The first method is to use the products template spreadsheet. The easiest way to do this is to sign into the WebManager and click on Manage Data > Export Data, then click on "Export Products". This will download the product template spreadsheet with all of your current data on it. You can either scroll down to the bottom and add your products down there, or you can erase everything so that you have blank template to work with.
You can upload the spreadsheet back into the WebManager to add those products once you're finished, as outlined in this article.
2. The other method is to sign into the WebManager and click on Products > Products Quick Edit, then click on the plus sign symbol in the top right corner of your screen. This will allow you to add a single new product into your account. This article explains the function in more detail.