I use various services that offer 2-factor auth through various iPhone apps. When I go to log in, I have to poke through my apps until I find the one for DO. It would be handy if the "enter your token" screen mentioned that DO uses Authenticator.
You should actually be able to use any app that operates off of the same library. Some of the issues that customers encounter are related to time-syncing and how the specific app they use handle that, but you should be able to use other auth apps, not just Authenticator.