It isn't shown in the episode so this isn't based on evidence, but I'm guessing with a very simple mechanism, probably something basic such as a bit of rope attached to the back of the fridge that he could use to pull it backwards once he was inside the vent.
If I remember correctly the vent was also quite wide at the entrance, so in a crouched position he could probably quite easily have grabbed it by the sides/back and pulled it with his hands without any sort of mechanism.
The fact that Luther throws the fridge aside with such ease probably indicates that it was empty, thus wouldn't have been very heavy or difficult to move. It also looks like an old fridge, so perhaps it was not one he used for food and was simply a decoy to cover his secret exit, therefore would have been stripped of the heavy motor etc. in order to make it easier to move into position.
It didn't need to be a very technical setup, so long as the fridge covered the hole he escaped from. Considering the trap he had rigged in the fridge in his victim's apartment he would have had plenty of time before anyone attempted to investigate it, as the bomb squad would have needed to get there and then they would have spent a lot of time attempting to secure it against a possible explosion/ evacuating neighbors etc.
This means that the fridge didn't need to be meticulously placed in order to conceal his exit, as he assumed no one would have even approached it, never mind tried to move it or look behind it.