To use Keybase in Mutt, the first thing that must be done (before even the macro) is setting your editor. This is important because we can replace the editor with any command we could think of. Mine looks like this.
set editor = 'echo %s > ~/.mutt/keybaseMutt/.tmp; vim %s'
It is crucially important, without this line nothing will work.
What is really happening in that line is this. The echo command is echoing the contents of %s into a file. Typically, an editor will use %s to open the temporary file to let you create an email. We've just used it to work with a python script later on.
Next, set a macro in the .muttrc