You need to add it in the shipping options when you're filling out a listing.
Under the services you've chosen for shipping there should be a line underneath saying "Apply my x shipping rule with profile name:" with a checkable box beside it, or should say "No x shipping discount rules have been created." with a clickable "Create rules" underneath. (Where x is either flat or calculated depending on which one you're using.)
Hopefully that helps 😆
