Welcome to the forums!

What version of b2evolution are you using?
Anyway, make sure you have the following settings at the end of /conf/_advanced.php

// ----- CHANGE THE FOLLOWING SETTINGS ONLY IF YOU KNOW WHAT YOU'RE DOING! -----
$evonetsrv_host = 'rpc.b2evo.net';
$evonetsrv_port = 80;
$evonetsrv_uri = '/evonetsrv/xmlrpc.php';

$antispamsrv_host = 'antispam.b2evo.net';
$antispamsrv_port = 80;
$antispamsrv_uri = '/evonetsrv/xmlrpc.php';