I just installed AsteriskNOW 1.5b and had the same thing happen. I added a SIP Extension using FreePBX and then ran “amportal restart” from the command line to restart asterisk and everything is working fine. I think that Asterisk does not load the SIP commands if there are no sip extensions. I don’t know if this is a bug or a feature, but…
Also, before i restarted via amportal, the output of the command “show modules like sip” did include chan_sip.so.
Just thought this info might be helpful for others searching for a solution to this problem.
Tom