Your merchant should first sign up for a PayPal Premier or Business account (if they haven't already).
Then, as far as ShopSite is concerned, go to the Commerce Setup -> Payment menu, check the checkbox next to the PayPal option, click the Configure button next to it and enter the email address the merchant used for their PayPal account, and click Save Changes at the bottom.
BTW, I would recommend v7 or newer for PayPal, because with earlier versions the PayPal integration in ShopSite did not use the IPN callbacks to complete the order, a new ShopSite order was generated as soon as the shopper was sent over to PayPal, whether or not the shopper completes the transaction once they are over there. See the warning in the v6.3 docs here that is not applicable in v7 and newer:
http://www.shopsite.com/help/6.3/en-US/ ... aypal.html
-Loren