Thank you for reply.
I did what you suggested. What I got was the iphone window run in the iPad simulator. The main window in MainWindow.xib is actually at the iphone size. Under the ~Library/SDKs/ArcGIS, I only saw iphonesimulator.sdk and iphoneos.sdk. Isn't the ipadsimulator.sdk supposed to be there too? ESRI said the sdk supported iPad. I hope it didn't mean that iPhone apps run inside an iPad device. I guess the AGS wizard created a new project with iPhone as the default.
Anyway, I created a view-based project for iPad and manually added each AGS component into the project. It worked. But is there any other way?