You have a few ways to assign keywords.
The Keywords window. Choose Window > Manage My Keywords. Select the photo(s), then click one or more keywords.
The keyboard. If you assigned a keyboard shortcut to the keyword, reap your reward now. With the Manage My Keywords window open, select the photo(s), and tap the shortcut key.
If you have to assign various keywords to a set of photos, combine shortcut keys with the arrow keys. Select a photo, tap a key, tap an arrow key to move to a new photo, tap a key.
In the Keywords box in the Information pane. See the tip on the opposite page.