In order to refresh the DNS cache in Mac OSX, execute the dscacheutil command with -flashcache as the parameter.
Usage: dscacheutil -h
dscacheutil -q category [-a key value]
dscacheutil -cachedump [-buckets] [-entries [category]]
Open terminal and execute the command
mini:~ ma$ dscacheutil -flushcache
This command will reload the cache and now you should be able to resolve any domain names that were previously not mapping to an IP or were pointing to an old IP.