The file you purchased is a compressed (zipped) file, the easiest thing to do if you have access to a computer is to download and unzip the file using that and put the contents into iTunes to transfer to your iPad.
To download it login, visit my account->files and click on the file (e.g., Simply Calm.zip).. when this is downloaded to your computer you should be able to unzip it using an unzipping program (free examples of zipping programmes are http://www.zipeg.com for mac or http://www.jzip.com for pc)
Alternatively if you don't have access to a computer I suggest using izip - a free app on the app store. One of our users said they used it and it worked a treat! :)