To test the Dropbox Sign API locally, you'll have to use a service to tunnel a public url to localhost (your local machine, as opposed to a publicly accessible url). Here are few examples of web services that allow you to do so:
ngrok
localtunnel
pagekite
forwardhq
These types of tools will give you a url to your localhost port, which you can use as the account and/or app callback URL while you're testing.
