Instead of just username on lines 16-18 there should be settings for username, password, and signature like:
$nextIndex = count($WA_PP_ECO_Do_required[0]);
$WA_PP_ECO_Do_required[0][$nextIndex] = "Username";
$WA_PP_ECO_Do_required[1][$nextIndex] = "api_username";
$nextIndex = count($WA_PP_ECO_Do_required[0]);
$WA_PP_ECO_Do_required[0][$nextIndex] = "Password";
$WA_PP_ECO_Do_required[1][$nextIndex] = "api_username";
$nextIndex = count($WA_PP_ECO_Do_required[0]);
$WA_PP_ECO_Do_required[0][$nextIndex] = "Signature";
$WA_PP_ECO_Do_required[1][$nextIndex] = "api_username";
$nextIndex = count($WA_PP_ECO_Do_required[0]);
These should be the API username, password, and signature... they are not the same as what you use to log into your paypal account.