Pro Tools Express won't install on new iMac

Hi.



I'm trying to install Pro Tools Express that came with my fast track duo on my brand new iMac. Only now have I realised that it is not compatible because of yosemite..



Is the only option to upgrade? And if so, what versions will work and can I upgrade without being able to install the version I paid for (Pro Tools Express)?



I'm obviously looking for the cheapest and quickest way because I've just spent a lot but I just want to get started.