1. Make sure you've patched to the latest version of the PayPal modules.
http://www.zen-cart.com/forum/showthread.php?t=56216

2. Go to your Zen Cart admin area and edit your PayPal Express Checkout settings. If you don't want to ensure that your customers only use a confirmed address, make sure "require confirmed address" is set to "no":
Express Checkout: Require Confirmed Address
Do you want to require that your customers use a *confirmed* address when choosing their shipping address in PayPal?
- Yes
- No
3. If there is still a problem, ensure that debug logging is enabled, do some tests, and post a link to the zipped log files.