You will find a checkbox  labeled as "[b]Upload new EJS jar file[/b]" when you edit the message.
It is located at the same line as submit button (labeled as "Post")-- it is al the far right side.

You will find a button to select file from your local file folder if you ckick the check box. (similar to upload file to server).
The applet will be shown following the end of the posted message automatically.

However, the ejs source has to be saved as ".xml" (it is not support .ejs source created jar yet).