Anil,
These are the steps I took to get this to work for me.
Create a new Web Project. Go to the project properties dialog (right click on the project and select Properties). Go to the Builders page and disable the JavaScript Validator (it will warn you but disable it anyway).
Go to the general MyEclipse preferences (MyEclipse->Preferences) and open the MyEclipse->Files and Editors->JavaScript page. Enable Active mode.
Now create the application, copying in the Extjs 4 SDK as the Getting Started guide mentions.
Content assist may be a bit slow to start with as indexes get built, but it should all work.
Please let me know if this works for you.