Either is fine.
Nakano-ku means Nakano. Each prefecture has
districts, and Ku is what that means. Nakano is
one of the 23 districts that is followed by "KU",
all other districts are followed by "SHI". Tokyo
is different from other prefectures. Its one of
the two, that uses "TO" instead of "KEN".
Tokyo-To Nakano-ku is the way you would write the
adderss if you're sending within Japan.
Tokyo Nakano would probably be the way we would
write it if you're sending to Japan.