At this point Symantec DLP does not have RADIUS authentication support, and it is not currently scheduled for an upcoming release. In most cases RADIUS is used to authenticate dial-up connections and authenticates against the Active Directory (AD) setup because companies want to keep it in sync. Therefore, you could use AD for authentication, but not RADIUS directly.
Here are some links to information on how to do the Radius-AD integration:
http://articles.techrepublic.com.com/5100-10878_11-6182945.html (Configure Cisco routers to use Active Directory authentication)
See KB 45690 for information regarding single sign-on. Some customers would like to enable logging onto the Enforce server via Siteminder or Tivoli WebSeal. However, single sign-on for Symantec DLP is currently not supported.