Im not able to confirm this until late tonight, but I believe you need to run the RetroPie-Setup script to enable the driver. I don’t think its loaded by default.

I don’t know where it is off the top of my head in the SD image (probably ~/RetroPie-Setup/somewhere)

but RetroPie-Setup/master/scriptmodules/setup.shinc has an installation script for the driver.

I am probably not helping, sorry, :P