Hello,
I am assuming here that you are trying the register the user as the DA. If that is the case, the confirmation email can sometimes go to spam. If they check their spam filter and cannot find it, have them try to log into the CSAM tool: http://tools.cisco.com/CDCEB
If they are unable to log in, update this post and we will reach out to you to investigate further.
Thanks!
Nathalie