You may already have UFORadiant depending on how you installed UFOAI and, if compiled, what options you compiled with. If you go to your installation directory for UFOAI, look for a subfolder named "radiant". If it's there, UFORadiant is already installed. Click on the uforadiant.exe file in the radiant subfolder to load it and skip ahead to the Mapping for Dummies tutorial.
If not, you'll need to download or compile UFORadiant. At the moment it doesn't have a regular release cycle or a "stable" version (although most versions are pretty stable). This means tracking down the right installation file is not always that easy, something we hope to do better on in the future.
For downloading UFORadiant, please visit the Download section.
You are encouraged to get the source from the Git repository and compile yourself.
Dependencies for compiling under Debian/Ubuntu:
compile the mapeditor with:
after you compiled UfoAI itself as described in Compile for Linux with:
The first time UFORadiant start, it will ask you for your UFO:AI installation folder. Just point it to e.g. C:\Program Files\UFOAI-2.5. Once you've got it setup, move on to the Mapping for Dummies guide.