This is the 5d housing, but the part seems to be generally the same. First, find a girl (pardon if you are one, I forgot to check before typing this!) with small dexterous fingers, you'll need them.
Generally, the way that doodad works is the spring wraps around the base of the shutter arm (inside the housing). The blunt end leans against the front wall of the housing, pushing and leveraging off of it, while the hooked end hooks around the corner of the shutter lever. The effect is to squish the spring when you pull the shutter release towards you, giving some resistance. Pretty clever, actually.
Just look at the spring and orient it correctly, then squeeze it a little, and slide it onto the shutter arm. Mine went on fine, but it took me a few tries to get my fingers on it correctly. Actually, a 10 year old might have ideally sized hands for this. Needle nose pliers aren't going to help, the space is too small. You gotta find somebody with good fingers.
So here are the pics:
1. Looking into the housing from the back:
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http://www.pbase.com/taxgeek/image/62358243
2. Same, slightly different angle:
http://www.pbase.com/taxgeek/image/62358244
3. From front top. Bottom of picture is the right side of the housing (as viewed from back), top of picture is left.:
http://www.pbase.com/taxgeek/image/62358245
Hope this helps.
Taxgeek