I do not have a manual and I need to attach the zipper foot to the Singer Merritt 2404.
First of all, download your free copy of the user manual from Singer:
That link takes you straight to the Singer page, key in your model number as 2404 in the search box and hit enter-that takes you to the download page where you'll see a free copy available. Click download and it comes up as a pdf copy you save to your computer. I have several of these (I teach sewing and there are a lot of vintage Singer machines out there) and they're great. They print off nicely and can be set into a binder.
You'll also see a purchasable book for sale. I've bought these before, they're not always the original manual but are a reprint in a spiral bound format. Worth the money if you need a hard copy and don't have a printer.
The info you need about the zipper foot is found on page 10, and pages 36-37.