the DataAssist insert record will sore the ID value in a session variable.
If the registration page is using the data assisat insert record behavior, note the session name in the store as section. Use this session in your email.
The wizard will create the registration page using the Dreamweaver insert record behavior, but the "Introduction to SecurityAssist" tutorial on the Solution recipe section of the Security Assist support page has steps for changing this to use the data assist insert record behavior:
http://www.webassist.com/support/securityassist/