Ok, so I'm posting to my own response. =o) The problem is that when you do a network install with the program files on a shared network server, some of the files get removed/not installed from the program directory. One of these files includes the ninfobox.dll file which is needed to open the Attachment Properties box. Thus you get the "File does not exist" error message. So basically find the file somewhere (in my case a fully installed client), and copy the file to the networked share. This problem was suppose to be fixed in 6.0.3 but I guess it wasn't. Hope this helps anyone having similar problems.
http://www-10.lotus.com/ldd/r5fixlist.nsf/a8f0ffda1fc76c8985256752006aba6c/15a704e791661bf285256d98004cf6b0?OpenDocument