I've searched around and found a lot of advice saying to modify the hosts file as such:
127.0.0.1 mydomainname.com
I've done that, but my actual site at mydomainname.com is still being used. Any other troubleshooting steps I could take? Any other suggestions?
simply setup your canvas address as localhost. Unless you're using FBML, it works. See http://fbgraph.computerbeacon.net/tutorial/section1/createnewapp.aspx for an example.