Dear Kathy
Though I don't know if you are using a bundled version (bSE) or a version aquired via online
purchase from our webstore (Internet
SE => iSE), I am quite sure that this isn't a version for your scanner.
I just check both installers which can be downloaded from our website and they both show the Astra 6400 as supported in the SF_Logfile.
So, if your SilverFast doesn't, just go ahead and download the version for the
Umax Astra 6400 (either bSE or iSE, depends on if your SilverFast came bundled with your scanner or not) directly from our download section and install it.
You should remove the SilverFast folder for the already installed, inappropriate version, from your /Applications/Photoshop/Plugins/Import-Export folder first.
Happy scanning :)