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（普通配置）' under 'Global Settings（全局设置）' in the left-hand menu:
Then you need to enable the JIRA RPC Plugin in 'Plugins（插件）' under 'System（系统）' in the left-hand menu:
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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.