Windows

Under Windows, there is an installer available at the following address:
http://sourceforge.net/projects/ama-ardoise/ (only a 32-bit version is available for the moment)

Note:
On a recent windows, the installer needs to be run with administrator rights. If you do not have these rights, or if you do not want to use the installer, there is also a zip archive which also can be used on a USB key.

Unix-like and MacOS

The cleaner way is to install through the package manager of your distribution if available (only Archlinux is supported with AUR at the 2013-10-07). Otherwise, you must compile the program yourself (do not worry;)). Most distributions offer Qt5 packages (install the development or "sdk" version) else, it must be installed via http://qt-project.org/downloads

Once Qt5 intalled, theres two ways: use git (easier to update from the previous sources) or download a tarball containing the sources.

Using git:
git clone https://git.gitorious.org/ardoise/ardoise.git git checkout master

Using the tarball:
Download https://gitorious.org/ardoise/ardoise/archive-tarball/master And extract it with your favorite archiver (7zip can do it).

Once the source code downloaded, follow the instructions in linux/INSTALL.txt

Note: I do not have Mac computer, so I cannot test the installation under this OS. If somebody is able to try, I will be very grateful he tells me about his experience here: http://sourceforge.net/p/ama-ardoise/discussion/general/