RT - When you add files to your project, they become compressed within the ebook.
When a reader opens the compiled ebook, it decompresses the content into an encrypted format in a temporary directory.
Thus, your Modal Dialog Box is trying to read an encrypted file; because it is it's own window, it isn't getting the file decrypted first.
As far as solutions go, I'm not sure if one can write to a Modal dialog on the fly, like you can for other windows; the only sure way I know of is to call an html page into your MDB from outside of the ebook.
I may be incorrect here, but I intend to work with it later and try some ideas....
best -