Adding Paypal button to existing page

I had a developer add a store and shopping cart page to our site. I have since decided to connect Paypal for a payment option. I have tried going through Paypal to get the code and copy the url to the page. When I go to the page it does not give me the option to add the url. I have also downloaded the plug-in but when I follow the steps to publish it on the page nothing shows. Anyone know what I may be doing wrong? Thanks!