Problem configuring Paypal module

[SharonS]

I'm using Prestashop V 1.4.1.0 and trying to configure Paypal V 2.4.

If I go to checkout and try to pay with Paypal this is the error message I get

PayPal response:
TIMESTAMP -> 2011-06-22T10:34:55Z
L_ERRORCODE0 -> 10002
L_SHORTMESSAGE0 -> Security error
L_LONGMESSAGE0 -> Security header is not valid
L_SEVERITYCODE0 -> Error
PayPal returned error


Can anyone point me to a step by step guide as to how to configure this module correctly.

Thanks

Sharon