The downloadable one is exactly the same, there is just a 30 day trial period before you will not be able to access it anymore.
To buy it, just download the trial version and when the box pops up at the beginning of the program asking if you wish to "Purchase now", "continue with free trial" or "Already paid", click Purchase now. They will send you to a screen where you fill out your info, credit card information and such. Then afterwards when you have bought it, they will send you or give you a validation, serial number so you can click on "Already Paid" and go through those steps.
Cheers