How do I download iTunes to my iPad mini w/ retina and sync to my PC?

You download and install the current version of iTunes on your computer, you already have a built-in iTunes app on your iPad (you can only update the built-in apps by updating iOS, and the iTunes app on your iPad isn't used for syncing).

iTunes for your computer : http://www.apple.com/itunes/download/

syncing with iTunes : http://support.apple.com/kb/HT1386

If you haven't synced your iPad to that computer before then you may also find this page useful : https://discussions.apple.com/docs/DOC-3141

If you can't find the iTunes store app on any of your iPad's homescreens or app folders, and you can't find it via the spotlight search screen (pull down the centre of the home screen in iOS 7), then is it hidden by Settings > General > Restrictions > iTunes being set 'off' ?

But you don't need access to the iTunes store app on your iPad to be able to sync it to a computer - it's iTunes on your computer that matters.

See if the troubleshooting on this page helps : http://support.apple.com/kb/TS1538