Once you have purchased the software license, you can download the FileMaker application from "My Account", when logged into the Computech website. Here is a link to my account (you will need to log in first):
http://www.computech-it.co.uk/myaccount/
When you have logged in, you will see an option called FileMaker Bundle Downloads. Click this link and you will be able to view your FileMaker files to download.
Example image:
When viewing the FileMaker bundle downloads page, click on the relavent FileMaker version for your operating system to download.