Farhan,
The Roadmap is only available at the Project level, not the Customer level which is why you do not see it. You can simply drill down into the Project from the Customer Work Plan where you are viewing the Projects. Drill down to the Project will bring you to the Project and allow you to see the Roadmap.
If you want the Roadmap available at the Customer module, this would be considered a Feature Request.
Sincerely,
Boris