How do I record vocal track while playing instrumental in headphones in Soundbooth CS4?

I want to record a vocal track while playing a backing track (Instrumental) in my headphones. I am using Soundbooth CS4. I can only record audio but I cannot play the instrumental. Thanks for your help.

What is a VPN?

VPN is (Virtual Private Network) were first used by companies to enable their employees to securely access internal systems such as email remotely (e.g. from home or while on business trips). Today they are increasingly being used for personal use by individuals to protect their privacy while online in public places (e.g. when using the wi-fi connection in a cafe) or in a country where the internet is censored / blocked (e.g. China, Saudi Arabia…).