I found that ibus did not work with Freya. Instead I used fcitx with mozc for Japanese Input, using the instructions from the elementary os subreddit.
NOTE: Make sure you remove the ibus package before you go through the folliwng steps, else the input-switcher will not work.
Add the Fcitx repository:
sudo add-apt-repository ppa:fcitx-team/nightly
You can change the keyboard layout in System Settings > Keyboard > Layout and add a new layout there with the plus button in the bottom left. I have no knowledge of Japanese signs, but enabling a Japanese (Kana) seemed to work for me.
The default shortcut to cycle through different keyboard layouts in Freya is AltSpace. You can change this in System ...
You can use xmodmap for that. First find out the keycode(s) of the alt keys using "xev". If not installed, you should install the x11-utils package. So type
$ xev | grep keycode
in terminal, focus to the small window by clicking on it and look to the output in terminal.
Now press the ALT key(s). You will see the keycode of the key and its name (e.g. ...
If you wish to use Japanese input follow the elementary subreddit here but with step (3) replaced with either
sudo apt-get install fcitx fcitx-anthy im-config
sudo apt-get install fcitx fcitx-mozc im-config
Depending on whether you wish to use (discontinued but widely used) anthy or (maintained by Google) mozc input.