In order to integrate Walmart Marketplace with your ShipStation account, you'll first need to be approved by Walmart to have your orders import into ShipStation. This approval process can take anywhere from a couple of days to a few months. To start this approval process, head here.
Once approved by Walmart, head to your Walmart Seller Center and select the gear in the upper righthand side of the screen. From the dropdown, select Consumer IDs & Secret Keys.
The page that appears will contain your Consumer ID and Secret Key (if you ever forget your Secret Key, you will have to regenerate it as it does not remain displayed after creation). Copy the ID and Key and make your way to ShipStation.
Head to your Account Settings (the gear icon in the upper right).
Next, select Selling Channels and then Store Setup from the sidebar on the left.