You need to copy and paste the list here:
WordPress Admin > Settings > Writing > Update Services
and click "Save Changes"
That's it.
Here is the list:
http://rpc.pingomatic.com/
http://ping.weblogalot.com/rpc.php
http://rpc.technorati.com/rpc/ping
http://ping.feedburner.com
http://www.blogpeople.net/servlet/weblogUpdates
http://api.moreover.com/RPC2
http://bblog.com/ping.php
http://rpc.weblogs.com/RPC2
http://blogsearch.google.com/ping/RPC2
http://ping.syndic8.com/xmlrpc.php
http://ping.bloggers.jp/rpc/
http://topicexchange.com/RPC2
http://xping.pubsub.com/ping
Disclaimer: This list is provided "As Is" with no warranties or guarantees. Do your own research before using it.