Remya,
Providing some clarification to Dipesh's answer. In OpenERP Menu Item can only call actions. There are 6 types of actions that I'm aware of: Window, Report, URL, Window Close (special action to close the current window), Client, and Server. So, you cannot, using the tools available in the OpenERP framework, call workflow transitions (we execute the workflow by executing it's transition) directly from Menu Item.
Now, you CAN execute a workflow transition through an ORM method (it can also be triggered through a button, but only if the active model and active ID of the view being displayed is defined. Since this would not be a straightforward process, I'll left this option out for the time being.) And ORM method can be called from a Server action of from a Button in a view displayed by a Window action. You can choose one of those routes (Server action or Window action) if you need to call a workflow transition from Menu Item.