Mitsukan is a common brand of
japanese rice wine vinegar (aka
su) that is used to flavor
sushi rice. It is available in most
oriental owned convenience stores like the
kwiki mart perhaps, yeah I know
apu isn't
Japanese.
It has a light and slightly sweet taste which makes it quite pleasant on the palate but for use in making sushi rice you will need to add sugar. The proportions I use that people have found to be most pleasant is basically a small bowl of mitsukan and as much white sugar as I can dissolve without heating it. Add a pinch of salt if you like.
The result will be a very light almost but not quite syrup-like solution which you should mix with your cooked koshihikari rice to make sushi rice. Ahh the joy of cooking.
Creds to gn0sis