I don't know the answer but I can guess that one of two answers is correct. When one has just the standard library, that library is located in the iPhoto folder that's in the user's Pictures folder. Obviously, since you have more than one library you must have more than one iPhoto folder. So:
Test Method One
However it is that you switch from one library to another, do so and start iPhoto. Now start up Pages and use the media browser. Do you see the same library as iPhoto does? Quit both programs and repeat with another iPhoto library. Important - change library, open iPhoto, then Pages. You may find that opening iPhoto isn't needed assuming this method works.
Test Method Two
if method one doesn't work it is because the media browser always expects the library to be the one named iPhoto in your user Pictures folder. That being the case, when you want Pages to look at a different library you'll have to actually move/rename that library so it conforms. Remember, you will almost certainly never be able to switch libraries without first quitting Pages.
thanks
in fact I found a way:
1 select the desired iPhoto library in iPhoto
2 opening media in keynote provides the same selected library
3 now found as well in pages
cheers f