zencart 1.3.9

I successfully installed zen cart on my server and i have configured my business paypal acct and entered the api info from paypal into zencart.

what i don't understand is how the express checkout integrates with zen cart and my site.

do i use the paypal express checkout button and somehow vector to zen cart

does my user see zen cart instead of paypal checkout - if so how to i set up an interface on my user's site to get to zen cart

i know i have 3 zencart php files that have to go somewhere but i don't understand where?

thank you very much!