How to use locate to search for folders only

Actually, locate has what it takes if you use the --regexp option and you don't mind it spitting out files that have the same name as the directories you seek. The "end of line" position marker gets the job done:

locate -r '/dirname$'

locate also supports --ignore-case if that's what you want.

Why not use the find command ?

find . -name YOUR_SEARCH_NAME -type d

locate itself can't do it for you. So the UNIX way to do it is to filter the output of locate:

locate --null something | xargs -r0 sh -c 'for i do [ -d "$i" ] && printf "%s\n" "$i"; done' sh {} +