Moodle 1.8 network possibilities do not work

[eckart]

I run a moodle 1.8 server with Plesk 8.2. Moodle 1.8 offers the possibility to connect with other moodle servers. But it requiers XML-RPC to be installed in PHP.
It seems to be that it's not installed in Plesk 8.2.

What can I do?