The default list of search pages is meant to be an example of how the add-on could be used.
You can add them manually in the Options page for the add-on or by using the add-on button from the Firefox menu bar.
If you wish to add them manually the addresses are:
https://www.newegg.com/Product/ProductList.aspx?Submit=ENE&Description=
https://www.bestbuy.com/site/searchpage.jsp?id=pcat17071&st=
http://www.microcenter.com/search/search_results.aspx?Ntt=