Happened to me recently
First up get a new one.
it goes from the bottom of the brake pedal forwards, in the box section should be a tab/lug about 4 or five inches fwd on the left hand side.
if youve got lots of rust flake in there then you may have to dig for it - and whilst there vacuum out any crud and spray it up with something.
Fitting is a pest - there are some good methods mentioned on here like with wire to pull the spring.
Basically locate the fwd end in the tab in the box, use some wire (i used an old wiper arm spring as it has a hook end) then use pliers to pull the new spring backwards to locate in the heel of the brake pedal.
Not sure migth have a couple of pics from when i did mine, will have a look shortly.