I am not really sure about old iMacs and Macbooks but all you need is QuickTime! My iMac (which runs Lion) has QuickTime version 10.1 and it is able to do this.
Here's how:
1. Run the QuickTime application. The application icon looks like this:
2. Go to its Menu, and simply click "New Screen Recording"
3. After you click this, and click on the Record button on the panel, you can either "Drag" with your mouse to specify an area, or you can click on the button to start recording the whole desktop. If you drag an area, there will be a button appearing at the center of the defined area "Start Recording".
The quality if good and enough for a youtube screencast demo of your app.
Here's an example of one I made just now (just 5MB) showing one of my app (AppliFX) demo.
(Note: this video is compressed by Blogger - captured one is a lot bigger and higher quality that this!)
