Comment 4 for bug 287316

It's in the standard installation. If you right click your panel and select "Add to Panel," you're taken to the selection box I have in the screenshot above. Double-click on "Pointer Capture," and you'll have added the green box on your panel. I put it under gnome-applets because it is a tool that can only be run on the gnome-panel and nowhere else - that's the definition of an applet [at least the one found in Help (yelp)].