How to implement field access rights

I would like to restrict access of a field to select groups of users. I know it can be done (see http://doc.openerp.com/trunk/developers/server/04_security/#field-access-rights), but I am not sure how to implement the code. I know the name of the field that I would like to restrict- how do I go about actually doing the change? I've tried under Settings->Technical->Security->Access Control Lists but have not had any luck. Any help would be much appreciated. Thanks, Lawrence