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 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.