Some Machines have a switch to lower the feed dogs on the back left side of the maqchine. Sometimes if you have a free arm on the machine you will need to remove it to find the lever. If it doesn't have one you can always tape an index card over the feed dogs. ( First make a hold for the needle to go through.) Hope this helped. :) Also you could always bring your machine with you for the store personel to fit with the foot. Stores that sell machines and feet usually are happy to help.
Liz