To apply a keyword to all your selected images using Apple Aperture 3, make sure to turn off the Primary Only mode using either the Edit-Menu, or the button in the right bottom corner that says [1]. It should be unselected like in the screenshot here below.

You can then use whatever method to add keywords, e.g. using the Control Bar (press D) and its Keywords Controls (press shift D).

This has been bugging me for a week. Maybe I pushed the Primary Only button by mistake. Yes, you can do it using a batch change. But luckily, you don’t have to do the simplest thing the hardest way. Just, careful where you push!