Reset DNS Cache on Mac

dscacheutil -flushcache is what you are looking for. There is a manual page on developer.apple.com that describes its usage.

I think that sending a hangup to mDNSResponder is the new solution to flushing the cache though - sudo killall -HUP mDNSResponder should do the trick.

Note This probably should have been asked on http://apple.stackexchange.com instead.