If you have an existing iPhone application that you want to upgrade to run on iPad devices you need to upgrade the target that builds your iPhone application into a target that can build both an iPhone and an iPad application, or add a target to your project for building the iPad application.

To upgrade an iPhone target for iPad development, select the target in the Groups & Files list and choose Project > Upgrade Current Target for iPad.

For details about iPad-application development, see iPad Programming Guide.

ref: http://developer.apple.com/iphone/library/documentation/Xcode/Conceptual/iphone_development/115-Configuring_Applications/configuring_applications.html