postfix – problem with multiple domains support as inarticle

hello, trying to setup postfix to support multiple domains thanks to Joel Rennich 's article, i am stuck with step 4: telling Postfix to re-map its user lookup tables. when i issue the command, i get an err message: "postmap: fatal: open /etc/postfix/virtual.db: Permission denied" The weird thing is i don't see any virtual.db directory in the postfix folder !! what am i doing wrong? thanks, alexandre