Step-by-step:
1.) Click your Start Orb
2.) Click Control Panel
3.) In "Category" mode, click "Change Keyboards or other input methods" below Clock, Language, and Region
4.) On the small screen that pops up, click the "Change Keyboards" button
5.) Near the middle-right side of the next screen that pops up, click the "Add" button
6.) Find Japanese, click the "+" sign next to it.
7.) Click the "+" next to "Keyboard" and check the checkbox next to Japanese
8.) Click OK.
9.) On the next screen, change your default input language at the top, and once again, click Ok.