Is it possible to use client object model in list forms
(editform.aspx,newform.aspx) using sharepoint designer, if yes howto?
Yes, you can access Client Side Object Model on list forms. You can add either Content Editor Web Part (as most people recommend) or open the page in SharePoint Designer and put your code on the page itself (many people take this approach).
Also I recommend using SpServices because it greatly simplifies your code in many situations.
This page gets your started with SpServices.