You should print  lexicographically  kth sequence  for length n
like for n=3
we have,
000 -- 1st
001 -- 2nd
010 -- 3rd
100 -- 4th
101 -- 5th

and for n=4
0000 - 1
0001 - 2
0010 - 3
0100 - 4
and  so  on..  you  find kth sequence for length n  :)
Hope it helps > :)