Download

You can download Kawaii Emoji Messenger for free.

Windows

You can download the Kawaii Emoji Messenger installer for Windows XP/Vista/7/8 from here.

Linux (source code)

To compile Kawaii Emoji Messenger from source code you need to download the tarball or clone it on GitHub.
Moreover you need to have Qt4 and cmake installed on your system, enter the source code directory and run the following commands:

mkdir build
cd build
cmake -DCMAKE_INSTALL_PREFIX=/usr ..
make
sudo make install

Ubuntu

PPA repositories for Ubuntu 14.04, Ubuntu 13.10, Ubuntu 13.04, Ubuntu 12.04, Ubuntu 12.10, Linux Mint and other derivatives.
Thanks to UbuntuHandbook.com:

sudo add-apt-repository ppa:ubuntuhandbook1/apps
sudo apt-get update
sudo apt-get install kawaiiemojimessenger