I'm not sure how determined you are to use the Paypal shopping cart itself, but if you're just looking for a shopping cart solution that uses Paypal as the commerce portion, you might give the Market theme a try.
I'll try to keep the hype to a minimum, but basically it's a premium theme that contains a built in shopping cart system. Just upload the theme, activate it, enter your Paypal email address into the settings area, and you're up and running.
The download package also contains a backend product manager plugin that you can use to add or edit your products through the WP administration area.
There's an online demo available at the Market website if you want to test it out before you buy.
I hope this helps.