While McCain thinks that he can lead an international coalition to impose sanctions on Iran, European countries tend to want to go through the UN Security Council first. The European Union follows UN sanctions and strengthens them, but many in Europe have business interests in Iran, which make bilateral sanctions much harder to do. Obama is correct in saying that meaningful sanctions would have to include Russia and China.
McCain quotes Obama as saying he would sit down with Ahmadinejad without preconditions. Obama did originally say that he would sit down with "Iranian leaders," though he has more recently talked about tough diplomacy, which could start with low level talks. And he pointed out correctly that the Bush administration recently sent a State Department official to a mutli-lateral meeting that included Iran.