i just installed Lightroom5 and imported some folders from Picasa. now when i try to develop photos from a particular folder Lightroom says that it can not find the folder. and the folder is missing from Picasa. but there are images from the folder which show up in Lightroom
Check the thumbnail in the library module. If there is a "!" in the corner, the image cannot be found on the hard drive.
If you right-click on the thumbnail, you should find an option to "open in finder/explorer".
The Folders tab in the left pane should show you where the image is supposed to be. You might need to "show parent" via right-click to see the whole directory tree.