You're going to have to set the nameservers on downdoggy.com.
Unfortunately, I don't have any information specific to that site.
Here's how to do it at GoDaddy:
http://tutorials.epilogue.net/web-design/how-to-set-up-the-nameservers-for-your-domain
That probably won't help you. But, maybe.
What I would do is e-mail tech-support at downdoggy.com, and ask them how to modify your nameservers.