How?
Start it. Start clicking. Nuku will start in Quiz Mode the first time
you start it, the mode in which you click upon the translation of the
character shown in the upper left corner. It will measure your score,
tell you when you are wrong, and in that case, what you did wrong. This
is the primary objective of Nuku. The other mode is Learning Mode, where
you simply click on the translated syllable you are interested in, and
Nuku will show the corresponding Japanese character.
...
Romanization systems?
There are several methods of romanizing Japanese kana, the collective
word for both hiragana and katakana. The three main ones are Hepburn,
Kunrei and Nihon-shiki. The official system is Kunrei, which does not
correspond to the actual pronounciation of the characters. However, this
is the main advantage of Hepburn, it is less regular than Kunrei, but
you can also tell how to pronounce words from the romanization. The
third system, Nihon-shiki, is basically Kunrei, slightly altered to not
map two kana with the same sound onto one romanization.