Lightroom 4 Keywords
One other way to add keywords to multiple images is to select them all in the filmstrip and then drag them (click down somewhere inside one of the images, not on the image border) to the keyword in the list. Of course, that adds only one keyword at a time, but it does it quite easily to multiple images.
When searching, try going to your "All Photographs" heading and then doing the search. If you need to know where a particular image is once that's done, RIGHT click on the image and you will be given a option to go to that image's folder.
Hope the additional tips help.