Cubase AI4 with Yamaha ME 8CX mixer

I bought a Yamaha ME 8CX usb mixer a couple of years ago that came with Cubase AI4. I did not install the software as I was using the mixer for other purposes, and since then have lost the original disc. Is there any way of purchasing replacement software, or something similar. I have Cubase 4 - 4.0.1 but it doesn’t recognize the mixer when connected. I use an old macbook pro with OSX 10.6.8. Any help much appreciated, thanks.