After some digging around the website I found the email they sent, it does indeed say version 6 for free. But you have to jump through some hoops to get it. The simplest way is probably to download version 6 from the link below and install it. (copmom that's probably the email you got; the 'full version key' link is broken.) Then go to the Internet menu in Photo Commander 6 and select "Switch to full version for free". Then you sign up for their newsletter and they send you the code. I have done it and it works! This is all straight from Ashampoo's website, not a third party. I got to the link below without being logged in, so it's no secret, either. I hope this helps!
http://r.ashampoo.com/mailings/1/090330_en.htm
-Duane.
p.s. my emails were in German for some reason, but the clicks required were fairly obvious. Your mileage may vary :)