Upon successfully launching the Add-On Portal from within Foundry, you will see a search bar in the upper right-hand corner of the screen (screenshot below).
The best way to search is with the Trading Partner ID (AKA: ISA05 on Purchase order mapping specifications). This is the ID which your trading partner has indicated for you to trade against.
You can also search on the trading partner name. Please be aware that many trading partners will have multiple results based on their name as they can have several different trade accounts due to different divisions and localities.
Simply enter the trading partner ID (without the qualifier) or the name and hit enter to receive the results list.