This can be done with native osx tools as well. Here's a snippet I created once:

  # First add a rule for all local traffic to port 80 to go into pipe 1
  # 100 is the rule number which will be used for referencing the rule later
  sudo ipfw add 100 pipe 1 ip from 127.0.0.1 to 127.0.0.1 dst-port http
  
  # To display the rule use
  # sudo ipfw show 100
  
  # configure the settings of the pipe as you please
  # 50kbit/s bandwidth
  sudo ipfw pipe 1 config bw 50Kbit
  # 200ms lag
  sudo ipfw pipe 1 config delay 200ms
  # 20% random packet loss
  sudo ipfw pipe 1 config plr 0.2
  
  # To display the current connections on the pipe use:
  # sudo ipfw pipe 1 show
  
  # when finished testing don't forget to delete rule and pipe
  sudo ipfw delete 100
  sudo ipfw pipe 1 delete