Do you have a camera? One isn't required to use IOCOM Visimeet, but is required to send video.
If you have a camera, is the light on the camera (signifying it is active) on? Do you see your video on the desktop? If no:
Go to Tools | Options | Video and confirm a camera is listed there and enabled.
Confirm that no other applications are trying to use the camera at the same time that you're in the meeting.
If another application is running, quit the application and restart Visimeet
The other site may not be displaying your camera. They need to right click the red camera next to your name on the participants list and select your camera to display your camera.
If the other site is using a Polycom, Tandberg or other video conferencing device, they can only receive one video from the conference. This video is selected by selecting a video image and pressing the "1" key on the keyboard. Alternatively, you can right click the video image, select "Selective" and click on "1".