Hello mainam,
If you are working with 10.1 ArcGIS server then I fear that module is removed from 10.1 . It was present with predecessor that is 10.0 ArcGIS server.
Hello Mainam,
You can open 'create web application tab' from ArcGIS Web Applications Manager 10.1 site from this url:
http://<Server Name>/<Instance Name>/Manager/login.aspx
Like
http://myserver/ArcGIS/Manager/login.aspx