To invoke JIRA operations remotely, you should ensure that the RPC plugin is enabled on the JIRA installation you are targeting. If you simply want to create a client to http://jira.atlassian.com/ then you can skip this step. First you need to check if the Accept Remote API Calls has been enabled in 'General Configuration' under 'Global Settings' in the left-hand menu:

Then you need to enable the JIRA RPC Plugin in 'Plugins' under 'System' in the left-hand menu:

(warning) If the plugin does not appear as above then your RPC jar has not been properly installed. Download the jar from the repository and copy it to the atlassian-jira/WEB-INF/lib folder of your JIRA installation. Perform a restart and your plugin should appear.

Your server should now be ready to accept remote procedure calls.