After changing to Japanese, look at the bottom right of your computer at the Taskbar. There should be a small JP to the left of your clock. To the right of "JP", should be some more buttons, a small keyboard, a red ball and the letter A. Click the letter A, and you should options for Hiragana, Katakana, etc. Click the one you want and continue on your way.
I have to do this everytime I want to write in Japanese.... I haven't figured out how to set Hiragana as the default...