Recording while tapping the beat is great if you don't want to stick to a rigid tempo as you record. You won't want to hear the metronome or any other music for that matter as you record while tapping the beat, because the metronome and the music will proceed in real time and you will not.
Select the Metronome dialog by clicking on the Option menu item and choosing Metronome. You want to make sure the metronome doesn't play when recording so remove the check mark in the box beside Metronome on Record by clicking on it. Click on OK.
Now select Record Options from the Options menu. Click on Tap Beat in the top left-hand part of the dialog. Put a check in the Listen to MIDI box at the bottom of the dialog by clicking on it. Now press the lowest note on your MIDI keyboard. The Listen to MIDI button will clear and the number of the note you pressed will appear in the Value list box. Click on OK. Click on the record button in the main control area. Now you are ready to record.
There is no hurry about entering notes. Remember to tap the beat with the left hand on the lowest note on your keyboard as you play with the right hand. When you're finished, press the SPACE BAR. Answer Yes to the question "Keep this take?" if you like what you did.
Remember it doesn't make sense to record while tapping the beat with your music playing. If you want to record while tapping the beat into a track with music either on this track or on tracks above or below it, it is best to mute the tracks. Do this by selecting the Track Sheet and removing the circles in the Play field for these tracks.