How do you add context senstive menu to NSOutlineView (ie right click menu)

In your menuForEvent method you can find out which row the click occurred on. You can pass that as a parameter to your defaultMenu method -- maybe call it defaultMenuForRow:

-(NSMenu*)menuForEvent:(NSEvent*)evt 
{
    NSPoint pt = [self convertPoint:[evt locationInWindow] fromView:nil];
    int row=[self rowAtPoint:pt];
    return [self defaultMenuForRow:row];
}

Now you can build the menu for the row you found in the event...

-(NSMenu*)defaultMenuForRow:(int)row
{
    if (row < 0) return nil;

    NSMenu *theMenu = [[[NSMenu alloc] 
                                initWithTitle:@"Model browser context menu"] 
                                autorelease];
    [theMenu insertItemWithTitle:@"Add package" 
                          action:@selector(addSite:) 
                   keyEquivalent:@"" 
                         atIndex:0];
    [theMenu insertItemWithTitle:[NSString stringWithFormat:@"Remove '%i'", row] 
                          action:@selector(removeSite:) 
                   keyEquivalent:@"" 
                         atIndex:0];
    // you'll need to find a way of getting the information about the 
    // row that is to be removed to the removeSite method
    // assuming that an ivar 'contextRow' is used for this
    contextRow = row;

    return theMenu;        
}

Also, as already mentioned in the comments, you really shouldn't use the NS-prefix on your own classes. There is a potential for a clash in the future plus it will confuse everybody that is looking at your code - including yourself :)

Hope this helps...

Here is a Swift 2.0 example which uses a subclass and extends the default NSOutlineDelegate so you can define your menus in the delegate.

protocol MenuOutlineViewDelegate : NSOutlineViewDelegate {
    func outlineView(outlineView: NSOutlineView, menuForItem item: AnyObject) -> NSMenu?
}

class MenuOutlineView: NSOutlineView {

    override func menuForEvent(event: NSEvent) -> NSMenu? {
        let point = self.convertPoint(event.locationInWindow, fromView: nil)
        let row = self.rowAtPoint(point)
        let item = self.itemAtRow(row)

        if (item == nil) {
            return nil
        }

        return (self.delegate() as! MenuOutlineViewDelegate).outlineView(self, menuForItem: item!)
    }

}

No need to subclass, its quite simple, and you can even customize the menu on the fly.

Declare an empty menu, set its delegate and set it on the outline views .menu property. As an added bonus this method works on both, outline and table views the same.

class OutlineViewController: NSViewController {

     private let contextMenu = NSMenu(title: "Context")
     
     override func viewDidLoad() {
        super.viewDidLoad()

        // other init stuff...

        contextMenu.delegate = self
        outlineView.menu = contextMenu
    }
}

extension OutlineViewController: NSMenuDelegate {

    func menuNeedsUpdate(_ menu: NSMenu) {
        // Returns the clicked row indices.
        // If the right click happens inside a selection, it is usually
        // the selected rows, if it appears outside of the selection it
        // is only the right clicked row with a blue border, as defined
        // in the `NSTableView` extension below.
        let indexes = outlineView.contextMenuRowIndexes

        menu.removeAllItems()
        
        // TODO: add/modify item as needed here before it is shown
    }
}

extension NSTableView {

    var contextMenuRowIndexes: IndexSet {
        var indexes = selectedRowIndexes

        // The blue selection box should always reflect the returned row indexes.
        if clickedRow >= 0
            && (selectedRowIndexes.isEmpty || !selectedRowIndexes.contains(clickedRow)) {
            indexes = [clickedRow]
        }

        return indexes
    }
}

Much later than the OP question, but for others like me wondering, here is my solution. It also needs subclassing NSOutlineView, which is not encouraged by Apple doc, anyway…

Rather than override menuForEvent: I override rightMouseDown:

- (void)rightMouseDown:(NSEvent *)event {
    NSPoint pt = [self convertPoint:[event locationInWindow] fromView:nil];
    NSInteger row = [self rowAtPoint:pt];
    id item = [self itemAtRow:row];
    NSMenu *menu;
    //set the menu to one you have defined either in code or IB through outlets
    self.menu = menu;
    [super rightMouseDown:event];
}

This has the advantage of keeping delegate calls to update the menu thereafter and also keeps row outlining on right click.