There seems to be a problem with photoshop and large TIFFs with 
packbits compression (which is used by panotools). Go to libtiff.org 
and get the tiff tools. There is a command line program enclosed, 
called tiffcp that can read the files an save them uncompressed or 
LZW compressed or what ever you want.
--Erik

Once I knew what the source of the problem, I discovered another
solution:  I opened the TIFF files in Picture Window Pro, which I had
already purchased and installed on my computer.  After saving
them from Picture Window, PS could open them just fine.