Manage Custom Sizes is greyed out
I would like to make a page to a custom size. This worked for me before, but now when I open a new page and then go to Page Setup, I see that "Manage Custom Sizes" is greyed out and I can only select from 4 or 5 different options (the main ones such as A4 / US Letter / US Legal etc....
What can I do to create and select a custom size?
Thanks!
Macbook, Mac OS X (10.5.6)