Add Click Track

The Recording menu's Add Click Track can be used to add a click track (metronome) to the song.

Add Click Track window

Length is the length of the click track in minutes.
Accent on Beat One increases the volume level of the first beat in every bar.
Instrument is the Drum Instrument to be used.
The Synth slot can contain any synth. If you plan on adding a drum track later using an editor, then it's a good idea to use the synth you'll be using for the drums in order to avoid any timing differences between the synths.
The Tempo section specifies the tempo to use. Current uses the current tempo, this option can only be selected if the song contains at least one MIDI track. Using the BPM option you can specify a new tempo. The Tap button pops up a 'BPM Tapper' window, which lets you play the tempo on the space bar.

Click the Store as default button if you want MultitrackStudio to remember the current settings.