To deploy a plugin you only need to copy it to the RHQ Server's <home>/plugins directory. It will get detected within a minute or two and processed. Ensure there are no errors in the server log. If it seems successful then there are two ways to get it to the agent. You should not copy it directly into an agent's plugin directory. One way is to restart the agent. Another is to execute the 'update all plugins' operation from the RHQ Agent resource (via the GUI), assuming you have already imported that resource into your inventory.
While developing a plugin it can be easier to validate and test it using the standalone plugin container. This is a lightweight plugin test environment that can be easier that repeatedly deploying the plugin into a full environment. See http://sourceforge.net/projects/rhq/files/rhq/standalone-container/.
Thanks for the info. I try o download the rhq-server.zip. It should be more than 350MB. But it is broken very often.