First of all I suggest you stopped using the Q3Support library (Q3PopupMenu - why not use QMenu?).

I see you misunderstand the concept of actions and menus. I'd suggest something like this (this is not a complete code!):
Qt Code: 
  1. class MyClass : public ... {
  2. //...
  3. private:
  4.   QAction *m_addAction;
  5.   QAction *m_removeAction;
  6. };
  7.  
  8. MyClass::MyClass() {
  9. //...
  10.   m_addAction = new QAction("Add to playlist", this);
  11.   m_removeAction = new QAction("Remove from playlist", this);
  12.   connect(m_addAction, SIGNAL(triggered()), this, SLOT(addToPlaylist()));
  13.   connect(m_removeAction, SIGNAL(triggered()), this, SLOT(removeFromPlaylist()));
  14. //...
  15. }
  16.  
  17. void MyClass::playListMenu(QPoint &pt){
  18.   QMenu menu;
  19.   if(shouldAddToPlaylist()){
  20.     menu.addAction(m_addAction);
  21.   } else {
  22.     menu.addAction(m_removeAction);
  23.   }
  24.   menu.exec(pt);
  25. }
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You can also add both actions and simply disable or hide the one you don't want. You'll get cleaner code then. Although for the QListView I'd use QWidget::customContextMenuRequested( const QPoint & pos ) signal and QAbstractItemView::indexAt() to fetch the item under cursor, but I don't know what you're exactly doing so some other approach might be better.