It's included in RejNX "starter Pack" which can be found here on gbatemp.
Make sure that your NSPs go to sd:\tinfoil\nsp\ and have short names without blanks like "picross.nsp" and "picrossupd.nsp". Always have NX-Shell on your Switch so you can move and rename NSPs if tinfoil can't find them, that saved me a lot of time yesterday.