CsoundQt is included in the Csound installers for Mac OSX and Windows. So the easiest way to get CsoundQt for OSX and Windows is to download and install Csound. For Windows, this will also install CsoundQt automatically. For OSX, you will find a .dmg which contains CsoundQt. Install Csound first, then activate the Disk Image, and drag the CsoundQt icon to the Applications folder.
For Debian / Ubuntu, there is a recent CsoundQt package at Debian Unstable. You can download and install it in the usual way (dpkg -i).
The latest release of the sources and the latest OSX installer are at https://github.com/CsoundQt/CsoundQt/releases/latest.
Other (older) pre-built binaries and nightly builds for OS X can be found on the Sourceforge Files page. The nightly build server is kindly provided by the Institute for Computer Music and Sound Technology (ICST) at ZHDK.
It's also relatively easy to build CsoundQt.