To change tempo or set the metronome on or off, click the metronome symbol in the righthand bar of tools, underneath the Undo and Redo buttons. That will bring up a panel with a tempo slider and various options.
If you check the "tick divided beats" box the metronome will provide two ticks for each beat, using a different sound for the offbeat. In a compound meter a divided tick will be heard in three parts, with two offbeats.
Note that the miscellaneous tools palette also contains a "tempo change" tool that allows you to insert a change of tempo anywhere in a piece.