Hi and welcome.
Sorry to say so, but what a crappy design.
The shield actually uses 6 pins, D8-D13.
The other pins are free to use as you like.
But those guys “designed” some “standard” of their own, and people buying any of their stuff will be forced to use that standard, like it or not.
The standard is called “grove”, and for some reason only the guys at seeeeeeed can think of, it is about a 4 pin plug with 2 power wires, and 2 signal wires.
Now you need to get a hold of some “grove” connector(s), and connect them to the right connector on your shield.
Tip: try to fit an old fashioned PC CD audio connector, looks like that may work.
Connectors J20-J24 offer analog pins A0 to A5 (each connector shares a pin with the adjacent connector : ).
If you look real close, you can see that on this picture from the page you linked to (click to open in a new tab): .