There are two main ways to run Windows software on a Mac.
One is to use a Virtual Machine, which effectively runs a copy of Windows on top of OSX. Running two operating systems at once and a hypervisor ([eg. VirtualBox, VMware, Parallels] - the software that allows you to run Windows in a Virtual Machine ) is obviously much more resource intensive than just running one natively. This works ok for undemanding software.
The other is to install Windows natively on your Macbook, by partitioning your hard disk (or SSD) using Boot Camp so that you can choose either OSX or Windows when you boot your Macbook. This gives either operating system full access to the hardware and is better for hardware-intensive applications.
For RANT, you could follow the author's own advice here:
RANT on a Mac. You could also try
WINE, but you may find that RANT does not work, or is buggy - there might be a reason Oddsoft didn't recommend it (this is actually a third way of running Windows software - sometimes it works, sometimes not).
Unfortunately, the Macbook Air is not really suited to flight simming; it is a lightweight in the graphics and CPU department. It is certainly not impossible, but expect to have to dial most of the settings down to their minimum in order to make it flyable.
You will definitely be in for a very disappointing experience if you try to run this on a Windows-based virtual machine. If you
really want to run Flight Simulator, I suggest you install Boot Camp and then install a native copy of Windows.
What you really want is something native to the Mac. You want
X-Plane. You should try the free demo for suitability on the Macbook Air before making a purchase.