I'm not sure where yo are seeing these instructions you refer to here:
When I follow the WebAssist instructions to go to My Business Setup, that page loads
can you provide a link to the page or PDF document where you are seeing these instructions so I can look into it.
I nthe WA_Globals, the $WAGLOBAL_Paypal_User, $WAGLOBAL_Paypal_Password and $WAGLOBAL_Paypal_Signature settings are all supposed to use the API credentials.
for the $WAGLOBAL_Paypal_User, use the API User Name. The $WAGLOBAL_Paypal_Password is the API Password and the $WAGLOBAL_Paypal_Signature is the API Signature.