I have written a simple Helloplugin that gets registered properly and shows up on admin console plugins page. But I have no idea how to test it. How can use a jabber client talk to the plugin?
That depends on what your plugin does. If shows up in the admin page, then it is loaded, so it means it will do whatever action you told it to do on whatever event. If you told it to respond to IQ’‘s, then it will respond to those IQ’'s; if you told it to do something with Packets, then it will do something with those packets.
If you want your plugin to respond to someone simply sending text to it from an ordinary client, as if they were chatting with a human, you need to have it register as a component, and you need to write a processPacket() method for it that handles packets of the Message subclass. If you want it to appear as online, you also need to have it handle packets of the Presence subclass correctly.