In order to select the DCs that you want to check stock for with Essendant and SP Richards go to Administration > General > Vendor Distribution Centers double click any DC then highlight each DC individually that you want to show on your stock check and select move up to give it a priority and also check the box check inventory and update.
Please note for Essendant you may need to also speak to EDI support to make sure these DCs are available on your account.