The new structured keywords panel in PM 4.5 is certainly an improvement in adding keywords. But, unfortunately, I still find it slower than adding keywords in Bridge.
To add keywords to a selection of images, I've tried double-clicking on the keyword in the structured keywords panel. This then adds the selected keyword to the set of images, which can take a while if there are a lot of images in the selection, and I have to wait until it's finished before adding the next keyword. It should be possible to simply click on each keyword to add each one to the panel at the bottom of the window then click on the apply button to apply all the keywords at once to the selected images.
Cheers,
Paul