How to make a input field readonly witth Java Script?

Hi All,
  I created a lead assignment rule to assign to new user with rule criteria : Lead industry = "Insurance".
  This rule is active and there is only 1 active rule.
  When I create a new lead or update an existing lead on the industry field to "Insurance", the owner field of this lead is not changed to the assigned user in the rule.
What did I miss to do? Or what is the concept of lead assignment rule  using option assign to a new user or a queue?
Thank you for your help.