those small jumpers come with the audio expander and you need to use them to enable individual channels and (optionally) to increase the input gain. See here.
Electrets normally have two pins. The one that is connected to the metal case is ground (-) and should be connected to the Bela ground. The other one (+) should be connected to a Bela input. Additionally you need to power it with a 3.3k ohm resistor from 3.3V to (+). On modern Bela capes (Bela Cape Rev B2) there is a better (quieter) voltage source for powering the microphone, this is in the form of an unpopulated pad labelled "FB2 micbias", which provides a clean 2.5V source. You can use a 2.2k ohm resistor from this to (+) instead of the solution suggested above.
The image below contains a wiring diagram for the electret mic (for the simple case of using 3.3V), although there is plenty of other stuff going on there. You should only care about the stuff inside the blue line