Each Bosch Microphone Unit comprises of three parts:  the base unit, the gooseneck mic, and the battery.

To assemble, take the goosneck mic and plug it into the back as shown below.

Next, take a battery and align the three tab guides as shown below.  The battery lights will be on the outside.

With the tab guides aligned up, press the battery into the unit until it clicks.

See Also:

Powering The Bosch Mics On And Operation