Hi Greg,
If you are searching in Ariba Connect and are using our On Premise product Procure-to-Pay is called 'Ariba Buyer'.
When you log-in you should see 'Ariba Buyer' in the Product Summary table in the top left of the dashboard.
In reference to your original questions, as Deepak highlighted you will need to follow the steps to create the Web Service. You can then generate the WSDL file and obtain the URL.
Regards,
Mat Theobald