In order to place a navigation bar on your modal view controller in interface builder (and set up bar button items that call actions in your detail view controller), you need to go through a level of indirection (your navigation bar will be in one .xib, and the details of your detail view will be in a different xib):
create a xib file containing a navigation controller object, and set its root view controller to be your detail view controller that you want to display modally with a navigation bar.
add bar button items to the detail controller's navigation bar and hook them up to IBActions in your detail view controller object.
your detail view controller will need to be in a separate .xib file
create a "loader" object that just exists to hold the navigation controller iboutlet, and set it to be the File's Owner object of that xib:
@interface Loader : NSObject
@property (nonatomic, retain) IBOutlet UINavigationController *navVC;
@end
@implementation Loader
@synthesize navVC;
- (void) dealloc
{
[navVC release];
[super dealloc];
}
@end
Your xib file containing the navigation controller will look like this:

Make sure the navigation controller object is conntected to the "Loader" object's navVC outlet, and make sure the bar button items are connected to your detail view controller's desired IBActions.
Then you present this whole thing using this code:
Loader *loader = [[[Loader alloc] init] autorelease];
[[NSBundle mainBundle] loadNibNamed:@"ModalVC" owner:loader options:nil];
[self presentModalViewController:loader.navVC animated:YES];