Enable random signature in Entourage

• Setup your signatures in tools/signatures
• In the signature put a tick in "Include in random list"
• Close the signatures and go to tools/accounts/
• Double click on your mail account and go to the Options tab
  
• In the "Default signature" pull-down select "Random"
• Click "OK"

Now each time you create a new mail message it will select a signature from your Random list.