Not at the mac right now, so can't be real specific, but in the menubar, probably under "photo", you will find something that says "add keywords" (or something similar). That should open up a box under the title of each picture, and you just type your keyword in there. If you want to keyword many photos with the same keyword, under "window" in the menubar, you will find something that says "show keywords. You can open this, select all the photos you want to keyword, and then click on the preferred keyword. You can edit this window to add and/or delete keywords, etc.