Button problem

Well , hello!
I am making a flash project but i have a problem:ex:
For example , I have 2 movie clips : “Help” and “Contact”
and I have made a input text field “search” (var=search_field).
There i also a button “go_btn”.
I put on button that script

on(press) {
if(_root.search_field =='help')
    _root.Help._visible = true;
    _root.Contact._visible = true;

But here is the problem.
Either if a write help or anything else it shows me the Contact movie clip.
When i change the “==” to “=” it show me Help movie clip for anyone word.
Please help me .