Creating a Modal Dialog or Window in Cocoa Objective-C?

I need to create an modal dialog, which is to loaded from a nib file and should be displayed on a button click in the main window.

I can create a custom window in a nib file and load the custom dialog on button click, but it's not a modal dialog. I can switch back to the main window.

MyWindowController is the NSWindowController subclass. I used the code below to display my window in response to the button event:

MyWindowController *pController = [[MyWindowController alloc] 
                                   initWithWindowNibName:@"nibfilename"];
[MyWindowController showWindow:self];