it is "Create Side Slope"

If it is already in your Custom Ribbon Group or in the Menu ribbon you are changing, then it may not appear in the list as it is already assigned to the Ribbon you are working on.
Don't disagree that a search would be good here - but my guess is that that request will be relatively low priority because you don't use this function that often and changing it will not lead to big productivity gains like many other requests right now at least
Alan