Color the square with coordinates $(i, j)$ black if $i+j$ is odd and white otherwise, for all $1 \leq i, j \leq 6$. Looking at the black squares only, we note that there are six distinct diagonals which run upward and to the right, but that two of them consist only of a corner square; we cannot simultaneously place bishops on both of these corner squares. Consequently, we can place at most five bishops on black squares. (This can be achieved by placing bishops on $(1,2),(1,4),(6,1),(6,3),(6,5)$.) If there are five bishops on black squares, there must be exactly one bishop on one of the two black corner squares, $(6,1)$ and $(1,6)$ : suppose without loss of generality that we place a bishop on $(1,6)$. Then, exactly one of $(3,6)$ and $(1,4)$ must also contain a bishop, and there are 2 ways to place two bishops on the four remaining black squares that are not yet under attack. Thus, we have a total of $2 \cdot 2 \cdot 2$ possible placements on black squares. Similarly, there are at most 5 bishops which can be placed on white squares and $2^{3}$ ways to place them, so that $n=10$ and $k=2^{6}$. Finally, $n+k=10+2^{6}=74$.