If you know the field and the condition
this is how you hide it
Xrm.Page.ui.tabs.get('new_fieldname').setVisible(false);
if you are online with the 2013, you can click on the "..." that is on the menu of the entity form and select customise entity > click forms > forms properties.
there you can find the javascript
after add the javascript you need to publish it.
With visual studio you can use the developer toolkit to deploy it by just clicking "deploy"
I Hope I could help. If I have answered please mark as 'Answer'. If was just helpful, please vote. Thanks and happy coding! Bruno Lucas, http://dynamicday.wordpress.com/