https://player.vimeo.com/video/689755768?h=99b8ca71f0&badge=0&autopause=0&player_id=0&app_id=58479
Simply click in the box to enable, revealing the polygon leaves in your scene.
Tree before Use Leaves is enabled.
Use Leaves enabled.
Activate this, to use particles for your leaves, by either dragging an xpEmitter into the link field or by clicking on Add Emitter.
You can then access the settings in the xpEmitter itself.
xpEmitter added, with the resulting particles on the tree.
Video to demonstrate an animation created using xpEmitter.