I own the domain name, iamgabeaudick.com. It was my personal website, but I now want to use it as my tumblr domain. I've changed the DNS, and tumblr confirms that the domain is set up correctly.
But when I access iamgabeaudick.com, a blank "Index of /" page appears.
Any ideas why this is? Do I need to configure the ftp files in some way?
Your domain is working fine for me (I can see your Tumblr blog). It sounds like your DNS is just being cached, either by your computer, or by your ISP.
You probably just need to wait 24hrs.
DNS settings take some time to propagate and not all DNS servers refresh at the same time. Like @webdestroya says, probably give it a day. For what it's worth it hasn't updated for me and I'm seeing the "Index of /" at the time of this post, but since it worked for someone else it probably just needs some time.