API account username

Hi all
I am a newbie.
I have a problem that I am sure you clever lot can help me with.
I have a business paypal account, and I am trying to get pay pal express to work.
It seems I have to register first a API account.
I have done this at the paypal end it says the the IPN has been activated.
But when I can use the shopping cart right to thepoint of having to register and then I get.
We are sorry for the inconvenience. The PayPal account authentication settings are not yet set up, or the API security information is incorrect. We are unable to complete your transaction. Please notify the store owner so they can correct this problem. (10002)

Now what I done in the admin is IPN url http//my website name.co.uk/ipn_main_handler.php
The return is
http// mywebsite name.co.uk/index.php?main_page=checkout_process

Can someone HELP PLEASE I would be very grateful.

                                                  Warm Regards
                                                                     Cliff

1. Log in to your PayPal Business account.
2. Click the Profile subtab located in the top navigation area
3. Click the API Access link under the Account Information header.
4. Click the Get Started link under the Request API Credentials heading.
5. Click the API signature radio button under the Credential Type heading.
6. Complete the API Credential Request form by clicking the agreement checkbox and clicking Submit.
7. Write down the values listed next to API Username and Password
8. Select the value listed next to Signature Hash, copy it, and save it to a file or some other location. This is your API Signature.
9. Use the values from #7 and #8 to configure the API SIGNATURE settings in the PayPal Express Checkout module in your Zen Cart™ admin area.