I know this is late. However when I searched for the answer to this problem this page came up. So I feel compelled to help fix it.
call search(l:searchString)
call matchadd('Search', l:searchString)
Did not work for me. (when run from inside a function) It did higlight the words I wanted to search for but n/N wouldn't cycle between them. Also when I performed a new search the "l:serachStirng" pattern still remained highlighted. This answer on this link worked much better
Vim search and highlighting control from a script
Which gave me:
let @/ = l:searchString
then run
normal n
outside the funciton (so the highlighting is done immediately without the user needing to press n)