Once I pay for my mach3 how what do I need to do to install this on my computer in the garage that is not linked to the net.

You will get an email with a file attached. That file is your licence and all you need to do is place that file in the main Mach3 folder. If you are not connected to a network with the machines computer you can transfer with a usb stick orportable hard drive or anything similar.
Hood