Only the CS5 version of Adobe Illustrator allows for multiple pages.  If you are building your booklet in AI, choose the Flyer template that represents your page size, and add additional pages for each page you are printing.

As a reference, for an 8-page booklet including cover:

Page 1 = outside front cover
Page 2 = inside front cover
Page 3 = first interior page opposite the inside front cover
Page 4 = inside left page of center spread
Page 5 = inside right page of center spread
Page 6 = last interior page opposite the inside back cover
Page 7 = inside back cover
Page 8 = outside back cover