The MM rating is for the number of Beats Per Minute (bpm). The bar has 3, 4, 6, (the top number in the time signature) whatever. The bottom number tells you what TYPE of beat.

For a 3/4 say, at 80 bpm, set the metronome beat speed to 80bpm. The mechanical types (except perhaps the VERY cheapest!) had another indicator that set the number of beats to the bar - set to 3. Now the clicking will be at the 80 bpm, and the bell will signal the first beat in a bar. Adjust this however you want.

Modern non-mechanical ones have similar functions.