BBM is by far the greatest mobile messaging application. Here is a way to have BBM on your desktop computer, using a very simple method involving BlueStacks.
BlueStacks is an Android emulator, available for free to download at http://www.bluestacks.com/. Both Mac and Windows versions are at your disposal.
Simply install the application, and run the emulator. There is a search button, which will forward you to the Google Play store. Once there, search for BBM, and find the one by BlackBerry Limited (it will most likely be the first one to show up anyway). From the Play store, click install, and a few moments later, the app will be installed on your emulator.
You can either log in using your current BBID, or create another one. The app runs in the same way as the Android version of BBM, for obvious reasons.
And that is how you can have BBM on your desktop computer.
NOTE: N4BB will NOT be held responsible if anything whatsoever goes wrong at any stage of the process. Proceed at your own risk.