If you are like me, I sometimes bring images into my computer from outside of Lightroom. This can be an image that you edited in a program other than Lightroom such as Photoshop, HDR EFX Pro or Portrait Professional. If I do, I still save them to the correct folder in my photo hierarchy. For instance, if I create an HDR directly out of an outside program, I’ll then save it to my Lightroom/Photos/2013/06 folder. But if I go into Lightroom, the program doesn’t know these images exist.
If you added files to a folder outside of Lightroom, just go to that folder in your Lightroom “Folders” panel, right click on the folder name and the select “SYNCHRONIZE FOLDER”. Poof! Your images will be imported.