mhash is a php function, not one that is defined by cart. you should contact your host to have the hash library enabled.
alternately, you can edit the WA_eCart/checkoutScripts/WA_simlib.php file to use the hash function instead.
return (bin2hex (mhash(MHASH_MD5, $data, $key)));
return (bin2hex (hash(MHASH_MD5, $data, $key)));