My guess is that the switch only operates on the phase leads, which carry the electrical pontential. The second one, the ground lead, is probably running through the back of the box. In Europe, phase would be blue and ground brown.
You'd need to cut the ground lead and run a short wire out to one terminal of the new outlet and then connect the cut again, so you get like a T-connection. To the other terminal, you connect the phase lead, but keep in mind to tap into it before it reaches the switch, otherwise the switch would operate the light AND the outlet.
