The default fee amount should be 0.001 BTC, with a minimum of 0.0001 BTC
You should just be able to change the amount in the text field in Preferences and then press 'Apply changes'.
In English the amount is written, say, "0.001" but if you have your language as, say, French or Spanish it will be "0,001"
I have the decimal separator set to ",", however MultiBit (0.4.15) uses "." everywhere. (Windows XP with a customized en-US locale.)