First you need to download and install the Java SE Runtime Environment 7. Be sure to click the Accept License Agreement radio button. Then click on the download link for your particular operating system.
Finally, download the most recent version of BBCT by clicking the button at the bottom of this page. This will be a ZIP file. Extract the contents of this file to an appropriate location on your machine. To run BBCT in Windows, double-click on "bbct.jar".