The compiler is giving you the hint that the argument list is missing - all you should need to do is add the brackets to tell it that you're trying to call the function. So change it to
CDiscRecorder* discRecorder = this->ui->cbDrives->itemData(index).value<CDiscRecorder*>();
And it should work. That's quite a long line, might be cleaner to break it out
QVariant variant = this->ui->cbDrives->itemData(index);
CDiscRecorder* discRecorder = variant.value<CDiscRecorder*>();