I never even installed Bryce 5.5 and when I went to www.daz3d.com/bryce55 it too me to http://www.daz3d.com/i.x/software/bryce/-/breg55? and it said...
You have already registered Bryce
Congratulations on downloading Bryce 5.5
and had the SN.

How it know that because I downloaded from C-Net and I never register it or installed it yet.
Then I clicked on Available Downloads and there it is listed along with other free Bryce software.

But I downloaded 5.5 from Daz too because I bet the SN is for there download and not the one from C-Net or I am not sure.
I know the C-Net 5.5 was "Bryce5_5_Free_Setup" was 122 MB and the Daz "br_ap003_Bryce55" was 118 MB.