Comment 17 for bug 248324

If this problem is still under consideration, here is what I just did to solve it (I've already posted this to a thread about adding a keyboard layout switcher):

For those who want to add applets to the panel (such as the keyboard layout switcher) of a standard UNR 9.10 Gnome desktop, first right-click on the open panel (black strip at the top of the screen) and select "Remove from panel." After that, you can again right-click in the same space and select "Add to panel" to add any applet you like from the Gnome desktop.

As near as I can tell, this works this way because what appears to be the panel on an out-of-the-box installation is actually a working applet called Window Picker (you can find it in the "Add Applet" dialogue box). You must first remove this before you can get to the real panel and start adding other applets. Don't forget, though, once you've added the applet(s) you want (in my case, the keyboard switcher) and moved it/them into the desired location on the panel, to add either Window Picker or one of the other window selectors back to your panel. Otherwise you won't be able to see your running applications at all!