Rosegarden has two built-in metronomes, one for use during recording and one that can be switched on during normal playback. They work by sending notes to a MIDI instrument, and both metronomes use the same instrument, pitch and velocity information.
To configure the metronomes, use -> to bring up the metronome dialog. Here you can select a MIDI instrument for the metronome and choose the metronome's resolution (whether it should tick once per beat, once per subdivision of a beat, or only once per bar), pitch, and velocities. Properties such as the actual voice the MIDI synth uses for the metronome tick depend on the instrument selected, just as the voice used for a MIDI track depends on the instrument that track is assigned to. You can configure the instrument by assigning a track to it on the main window and selecting the correct program from the instrument parameter box.
The record and play metronomes can be switched on or off from the Transport window, using the metronome button. This button controls the record metronome when recording is armed or in progress, and the play metronome at other times. By default, the record metronome is switched on and the play metronome switched off.
Rosegarden also has a "visual metronome": the transport window can be told to flash brightly to mark each beat. See the Transport section for more details.