You can add products 3 different ways:
Search your active & visible products and select the product and configuration to add
Add by Product Exact SKU
Create a custom product.
Add via Search:
Note that our product search will yield the same results as what a customer would see on the front end of your store. Visible Products Only.
In this example, I've searched the term "mold" and it's pulled up a list of all our products that would show for that term. We'll list about 20 results in your dashboard. If there are more than that showing up, you need to dial in your search term, or try putting your term inside quotes. ie: "mold remover".
Add via Exact Sku:
We've added the same product in this case utilizing the exact SKU of the product.
Note the SKUs are optionional in BigCommerce, and there are 2 types
You can use this with the exact base product sku, and the product option SKUs
Adding via SKU is the best way to add products that are not visible in your store.