In the meantime I've got things running thanks to accellion.com's quick and friendly email support which can be contacted here:
http://www.accellion.com/support/index.html
They provided me with an installation guide that was (for some reason) not included in the virtual appliance. I suppose this PDF is not "top secret" so I have uploaded it here:
http://www.badongo.com/file/10874589
The temporary license key was also send by accellion.com email support.
Setting up the appliance was easy, web interface is quite straightforward. On our Draytek Vigor router we just had to forward port 80 and 443 to the LAN ip of the appliance.