Well shit! I ordered the whole Season 1 of Harley Quinn on Amazon Instant and there are no episodes available! WTF?!
Always check the actual list of episodes on Amazon Video, looks like they got the free preview up but the actual episodes are not streaming on anything other than the DC Universe service.
Getting a refund is pretty easy if you haven't watched something yet, and obviously you haven't watched any episodes because they are not available. I've had to do it a couple of times after accidentally ordering a show.
Or just wait, once the episodes drop you will get them, although they don't have release date info anywhere, so you might have to wait for it to come out on DVD/BD, that is usually when these service exclusive shows are available on Amazon/iTunes/etc...
Edit: A quick search shows it is supposed to be available on iTunes on 3/24, so that would probably be the date it also drops on Amazon.