Try opening up the plastic mech ("to open squeeze") -- I just opened up one of my Canadian CA mechs and can't tell if there would be space for a magnet in there (mine has a spring), but I would expect it to be right around the area labeled "to open squeeze".
EDIT: Turns out I do have a US CA mech in my box o' mechs -- the magnet is the round cylinder at the bottom when you open the first "door".