The USB Host pin connections are highlighted on the Metro image to the left. You will need a small piece of 0.1 inch male header, with 4 pins, to fit the holes.

You can cut header with diagonal cutters or break them with pliers or even your fingers. Just be sure to wear eye protection as they can fly when cut. 

Put the short end of the header into the holes in the Metro marked USB Host. 

If you are using solderless header then they are press fit into the holes. You will need some pressure to get them in if they are the Press-Fit version, pliers will be required. While they are designed to make electrical contact, you might want to solder them to be sure.

If using standard header, secure them with putty, blutack, tape, etc.

Turn the Metro over and you should see the header barely poking out of the bottom of the board. If the pins stick through a great deal you may have the header pins upside down, double check the short end is sticking into the board.

Solder the 4 pin "nubbins" to the board.

Turn the board over and remove the material securing the pins. Now there is a new 4-pin header. 

Get the USB Host cable and wire as follows:

GRD to Black

D+ to Green

D- to White

5V to Red