what other java do you have running on your page have you disabled this to make sure you dont have any java conflicts?
Main Index - PhotoPost Photo Gallery
We run version 6.23 right here on this very site and the flash upload works. I also run Photopost on my site with the flash uploader enabled and all appears correct. I use the latest Mozilla, IE, Opera and Safari browsers although most of my time is spent in Mozilla.
Please post a url and test login for us to look at your site and see if we can suggest anything further. Initial suggestion is reupload all files except config, header, footer or stylesheet files and disable any java you have added in the header etc.