The extract filter was removed in CS4. Adobe says there are better options. They prefer edge detection as described below.
Edge Detection: The Extract filter was left out of CS4, although it was available as a separate install. (Rumor has it that the new Edge Detection feature wasn’t ready for CS4 and was pushed back to CS5.) While the Extract filter got a lot of bad press, it was really due to misuse. Luckily, Photoshop CS5 does a better job than the filter. Selection of complex edges, such as curly hair, has always been a problem. The Edge Detection setting is now part of the Refine Edge and Refine Mask dialogs. The feature is fairly easy to use: make a selection or a layer mask, open the Refine Edge dialog (Select>Refine Edge), and use the Refine Radius tool (E) to brush over the desired area. (There’s an Erase Refinements tool [just press Shift-E] in case you don’t like the results and want to try again.) In addition, there’s a Smart Radius checkbox with a Radius adjustment slider to make selections even easier. This tool makes short work of some tough selections, but it’s not for every image. I found it worked great on some images, but not others. You still might want to bone up on using the image calculations for advanced selections.