I thought I might shed some light on the invisible zipper foot. I bought a Birch model from Spotlight and it came in a little plastic bag with four blue shanks and a rolling foot.
First I removed the standard foot from my Janome sewing machine (Sewist 521). I realised I still needed to remove the shank, which is the part that is screwed on, and holds the interchangeable feet. So with one hand holding the screwdriver and one hand holding my daughter, who just had to watch, it popped out disappeared somewhere. NO! I was about to kick myself but luckily I found it in the basket underneath my table. Phew.
The Birch invisible zipper foot comes with one shank already in place so you know how it's suppose to fit. You have to select the correct shank that fits your machine and slide it onto the foot piece.
I just tried each blue part on my machine and made sure it was wide enough to cover the metal leg bit and that I could fit the screw in it's side. Once you find the correct shank slide it into the paddle pop looking part and slide it to the middle. Screw it in place and then you can start using it as per usual. I've added some pics below so you know what I'm on about. Was that clear as mud?
* For a terrific tutorial on sewing an invisible zipper check it out here