To add a new subdivision, tap the "Add Subdivision" button above the sliders. On the next screen, you can choose whether to add a common type of subdivision (such as quarter notes or eighth notes), a preset subdivision (for example, Metronomics has a few basic claves and other rhythmic patterns included), or a custom subdivision that you can define yourself.
If you have a subdivision in mind that isn't included on the common or preset lists, you can choose to create your own subdivision. The format of the custom subdivisions is a fraction. In the fraction, the top number (the numerator) represents the number of occurrences of the subdivision during the number of beats represented by the bottom number (the denominator). For example, 1/1 would be equivalent to quarter notes, since 1 subdivision would occur for every 1 beat. 2/1 would be eighth notes -- 2 subdivisions per beat. 5/7 would be 5 subdivisions for every 7 beats.
At the bottom of the screen, you can choose a sound for the new subdivision. Once you finish adding the subdivision, you can always go back and change the sound (or any of the other settings) in the subdivision options by tapping its circle icon.
Tap the "Add" button in the top right corner to finish and add the new subdivision to your metronome.