I plan to add Unity iPhone Advance to Unity Pro, should I learn Objective-C and Cocoa to use Unity iPhone? Or Unity 100% hide Objective-C and Cocoa to developer?
It hides it pretty well but if you want features not offered yet, you will not be able to avoid using Obj-C, for example if you want to use an UI stuff, game kit, store kit and similar
I just purchase the iPhone Advanced 
But should I register the iPhone developer now? Or I can register later? I don’t want to pay $99 for the developer program before I finished my game.
Congrats! you are now Unity family 
You have to complete the iPhone Developer certificate process to get the certificate. Otherwise, you will not be able to test or run your app in device.
Can I test the app in Unity first? And I just open the sample project, how to simulate the touch screen with touch pad?
You can open the sample project but will not be able to play it or touch it. To do this you will require Unity Remote install in your device. Please check the Unity Doc for detail.
Thanks
-Shohag
Is that mean I must join the iPhone developer program in order to install the Unity Remote? I have an iPod Touch.
the unity 1.x remote is on the app store.
but if you ever want to test the build on the device, yes you must join the developer program (you will need to be able to do that as the desktop only gives you a restricted replication of the state on the device, especially performance and some rendering aspects differ significantly and the desktop also will never have the problems in relation to low memory and similar)
what you definitely will not be able to do is finish your game without getting it onto your device, so plan to join the program at latest 60% through, optimally earlier so you know early on what never will work at all
Thank you so much!
After install the Unity Remote from App store I can control the demo game.
Please go through in this thread step by step. You will get all of your ans.
http://forum.unity3d.com/viewtopic.php?t=20432
Thanks
-grinder