In the Milestone software, the value above the cameras feed in the Management Client is the value per frame in KB (Kilobytes). To convert it to Kbps, multiply that value by 8 (converts from Kilobytes to Kilobits) and then multiple it by the frame rate of the camera.
For example, if the value is 1KB and you are recording 15fps then your bitrate would be 120 Kbps (1KB * 8 * 15). It is going to be an approximation because if the preview is showing 1KB, that means it could be anywhere between 0.5KB and 1.4KB. Because of that, your bitrate would more accurately be somewhere between 60 Kbps and 168 Kbps if your framerate was 15fps.
Milestone's Express+, Professional+, Expert, and Corporate have performance counters that can be used in Windows Performance Monitor to show a more accurate bitrate of the cameras.