I go to the download page and get SLES12SP1 for i86_64. On the page I see
the registration code. (I am logged in to SUSE, if this is neccessary.)

After the install in a VM w/o registration (belongs to the proxy*) I try to
register with 'yast scc'. But I get a message similar to 'Unkown
registration code'.

How is the correct way to install SLES12-Eval including registration to get
the lastest patches?

*During the installation it is not possible to register because there is no
possibility to configure a proxy. Maybe this could be a remark during a beta
test ;-)