Adding a keyword to multiple images at a time doesn't seem possible. I attempt it as following:
1) click on an image thumbnail
2) hold shift and click on another thumbnail, so selecting several images
3) click on a keyword
However the keyword gets only set for the image clicked most recently. This makes the keyword feature very impractical when I want to say set the same keyword for all images in a directory. I suggest making all keyword manipulation apply to all selected images.