1 go to developer.paypal.com
2 register developer acct
3 register test accts (example one business and one for the buyer)
4 add any (free;-) money to buyer's acct
4 follow the set-up steps for the payment type you wish to use - Express or IPN
- if you go for Express - go to Profile -> API Access -> Request API Credentials and use them in Zen Cart in which you have to select "sandbox" for PP Express checkout

phew, did it a while ago and works
tried the IPN but sadly ...
too late... gotta go to bed
hope this helps...

cheers