In the new thread and reply windows, you'll see this button:

Clicking it will add the [ GOOGLE ][ /GOOGLE ] tags to your message (with no spaces).

To add a particular video to the message, you need to add the Google Video file name to it, so for instance with this video:

you use the Google Video button and add the 7431528359726651610 bit (forgetting anything like &hl=en-GB or whatever) between them so it looks like this [ GOOGLE ]7431528359726651610[ /GOOGLE ] with no spaces and what you get is this:

Google Video
ERROR: If you can see this, then Google Video is down or you don't have Flash installed.