How exactly are you trying to 'open' a file? Are you using the 'File > Open' or using the 'Folder' icon. They both will allow you to navigate or Look into your folder holding your photos. Try changing the 'Files of Type' to All files. By default it uses 'Common Graphic Files' which would show you your photos. What file extension do your files have? If they are a .kdc, this is the Kodac format and it should be able to read them for viewing.
In all honesty, Irfanview is not really the best program for watermarks as they aren't semi transparent and you have limited positioning with the text.
You can achieve the same results using Microsoft's Paint program.