Others have used PayPal advanced in eCart 6.
are you testing from your local computer? or testing using the live server? you wont be able to test payments using the local computer.
also, when you request API credentials, you will need separate API credentials for the sand box from the API credentials for the live server.
the URL that the ppadvframe.php file sets thew form to post to is set in the webassist/ecart/PP_PayAdv_PHP.php file:
$servURL = 'plugins/payflowpro.paypal.com';
$servURL = 'plugins/pilot-payflowpro.paypal.com';
go to WebAssist -> eCart -> Payment gateway credentials to set whether the sand box is used or not, and the API Credentials that are used.