Download the latest version of the Blackberry Desktop Manager if you do not already have it (see Resources).
Open the software and connect your phone to the computer using the USB cable.
Click "Applications" on the left and wait for the available applications to load.
Scroll to "BlackBerry Messenger." Click the plus-sign button to the right of it. This places it in the "Pending" section.
Click "Apply" to install it on the phone. The phone will reboot when the installation is complete. Do not disconnect your phone from the computer until then.