What is a "mechanical pad"?
The "mechanical pad" is where you attach the housing of the flex connector you are using.

The adapter board is not intended to have the flex soldered to it.

The idea is that you solder a proper connector for your flex on the adapter board.  Then, you solder connections to the provided through hole points.

This lets you connect your flex to a circuit, then unplug and reconnect it without destroying your flex.