Think I got the "Comma" branded one a few years back on my
MK1 Leon. it done the job and got ride of the horrid smell.
instructions may vary but when i done it I think it was something like this.....
you put the a/c full blast and the recirculation button on. then put the spray can on the floor of on of the footwells at the back of the car. press the top button on the can and close the doors. the car then fills up with the gas/mist and you leave it like that for about 10-15mins.
then remove the can and open the doors, the dash maybel be a bit damp but that's ok.... once this is done you a/c should not smell any more. the instructions should be similar to this if I can recall.