For the first beat frequency it is mandatory after entering the frequency to enter a space and some non-zero value for duration. Otherwise you are creating a non existent frequency that has no duration. Similarly it is pointless (but not an error according to the plugin) to omit duration for subsequent beat frequencies.
For example, these settings work:
I have made a note about this on the plugin’s Wiki page, but perhaps Steve could add an error message sometime if the first beat frequency field does not specify duration.
Gale