When you export a project into AU or VST, the "default user interface" is used. Even if you have a custom GUI both APIs require the default interface to be coded; it's how the client automates your plug-in. The RackAFX GUI designer code isn't compatible with XCode. On the VST side, you would need Visual Studio Pro with MFC and you would be bound to using the same version of MFC as I did to write it - VS2008 SP1 due to a Microsoft compatibility issue. For these reasons I decided not to mess with GUI stuff. There are several GUI platforms you could implement including PyQt and JUCE so I will leave it up to you to layout and design your GUi.

VST: I left comments in the code giving you the hook points for your GUI.

AU: When you export you get two versions, with and without a Cocoa GUI. If you use the Cocoa option you will have to implement the Cocoa object; I left a sample object and code and a simple GUI for reference with comments showing you where to hook into the GUI.