There does not appear to be any information in the HOWTO for idmap on how to setup and use idmap_ad. I was asked by (firstname.lastname@example.org) to put in this bugzilla for this.
Since Samba 3.0.25 (and the rewritten idmap backend code), there is a separate manpage for the idmap_ad module. Therefore I will close this bug report. Please reopen if it is still an issue for you.