So you have a G15 but cant use the LCD or G Keys because your on a mac? I recently discovered if you load the GamePanel for the G13 it will recognize the G15 and allow you to use both the G keys and the LCD display.
You can download the software from here. Once installed it should detect your G15 and apps should start running on it immediately.
I purchased a G15 a few years ago so I have the older G15 with the flip display, I’m not sure if this will work with the newer one. although checking Logitech’s page for the G15 they now say the new one supports OS X, Please let us know your experiences with this device.