Before mounting the PSU, check that it's voltage input is adjusted correctly for your location. The switch on the side of the unit should be set to your mains voltage. In the UK this would be 230v, in the USA it would be set 110v.

Fix the PSU to the base with four M4 x 6mm button head bolts.

An easy way to line up the holes is to place the PSU under the base and look through the holes from the top until they line up, and you can secure one of the bolts. The rest should then easily follow.

Note the orientation of the PSU, it should match the third photo.