First of all, make sure that the folder where your files are stored is selected. Despite rumours to the contrary, PSP doesn't catalogue every folder on your computer. In the folder list on the left expand "Folders" (by clicking the plus sign next to it), then click the "Browse More Folders" item and navigate to the folder holding your images. It might take a few minutes to catalogue all 600 images.
If you now see your folder listed, but you don't see any or all of the thumbnails for the images in it, click on the name of your image folder in the list on the left, then right click where the thumbnails should be and select "Refresh".
If you still don't see everything, it would be best to delete the thumbnail cache and Organizer database, and let PSP create fresh ones. To do this use the instructions at
http://corel.custhelp.com/app/answers/d ... _id/762173. Even though this article refers to a different problem, the instructions for deleting the cache are good. It is important to check for a PSPP XI database as well. X2 uses settings it finds in an existing XI database, and a very common cause of X2 problems is corrupted Organizer settings inherited from an old PSPP XI or Snapfire installation.
After you have deleted the old databases, restart PSPP X2. You will probably need to add the location of your image folder again.