when you sign up to use the express checkout servioce, you will be given aMerchant account password with Paypal and Merchant account signature with Paypal to use setting up express checkout. If you need details on signing for api credentials, contact paypal suport
the sample code you have from paypal wont do anything with the IPN post.
to set up an IP page, go to the bindings panel, click the plus and select eCart -> Checkout -> Checkout form response. From the gateway list select Paypal, IPN
you can then add behaviors for updating records or sending an email receipt to the IPN page and use the IPN Bindings to get info from the IPN post.
I strongly recommend using paypal express checkout over paypal standard, it is much easier to implement.
