If the Rack does not have a PPU fitted, the cradles should immediately be active, and a device should be placed into any cradle slot to charge.
If the Rack is fitted with a PPU, it might not immediately power the cradles. If so, check that the RCD is in the On position. The RCD can be found on the left-hand side of the PPU. Lift the flap of the PPU to expose the three components of the PPU. Lifting the flap of the PPU is safe as the live terminals are behind the face plate. When the RCD is switched on, a small click sounds, which is the ICL switching over to direct connection from its initial current limiting connection.
If the Rack still does not work, then a qualified electrician should inspect the wall socket and electrical supply.