I need help to find only one rateing!

I don't believe there is a way to search for content by its rating directly through Roku search.  There is a way to filter it for parental control, but I don't believe that will work for what you are trying to do.  It is best to look for content specifically by name or possibly some of the more sophisticated apps may have a way to filter content in that manner, but you would have to look within each individual app.