I am sure this is really obvious.

How do I go about associating a macro with a keyboard action.  I know
how it works when recording a macro (simply use the options to associate
the macro you are recording with the macro) but I cannot seem to see how
it works if the macro was created elsewhere or elsewhen and I now want
to associate it with a keyboard.

For instance suppose I import

Sub macro()
'Do some stuff
End Sub

And then want to activate it every time I press ^B.  How would I do
this?