I have exported photos from iPhoto to the desktop, then imported to lightroom. Now that I am done editing in Lightroom, I want to export to burn them. However, in the meantime i deleted the files from my desktop and now Lightroom wont export them...it keeps saying error. Any advice?
You can't delete files after you have imported them into Lightroom. That will cause this error (also renaming or moving the files will cause this error).
You should export the photos from iPhoto again, and put them in the exact same location as they were before. This will fix the problem in Lightroom.
Expanding on DJ's message...
Your images do not reside "in" Lightroom. They exist on your hard drive like all your other files. LR just references the files by path and filename. If you delete/move/rename the files on your hard drive, LR will not know where to find them without you telling LR where the new location/name is.