Echo what everyone else said although I like mine set hard so I only use 150ml of fridge cold water and put in the freezer to set. I use a hand blender as I wouldn't want to waste any stuck around the side of a large blender ;-)
I'm going to recommend something slightly different - it's always worked for me.
Put 150ml-200ml of water in the blender cup thingy - and put it in the freezer for 20-30 minutes. Obviously you don't want the water to freeze, but the ideal 'coldness' is where you see ice crystals just beginning to form on the surface of the water.
Put a scoop of mix-a-mousse in the packet of shake and mix the two together. Then pour the contents into the cold water and blend.
Now this bit is more important than you might think - you need to blend the stuff for at least a minute (my only disasters have happened where I've forgotten to blend it for long enough )
It'll set in 5 minutes or so if you pop it in the fridge (I have been known to pop it in the freezer when I'm really hungry!)
thanks everyone!! ill be trying all those tips. i made one tonight which was in the fridge for ages, didnt seem to set brilliantly so i stuck it in the freezer for a bit... i was edible but not particularly enjoyable!!