As Port Town enters the battlefield, you may reveal a Plains or Island card from your hand. If you don't, Port Town enters the battlefield tapped.
: Add or to your mana pool.
4/8/2016 You may reveal any land card with either or both of the appropriate subtypes. It doesn’t have to be a basic land. For example, you could reveal Canopy Vista from the Battle for Zendikar set to satisfy the ability of Port Town.
4/8/2016 Lands don’t have a subtype just because they can produce mana of the corresponding color. Port Town itself is neither a Plains nor an Island, even though it produces white and blue mana, so you can’t reveal one to satisfy the ability of another.
4/8/2016 If a Plains or Island is entering the battlefield from your hand at the same time as Port Town, you may reveal the other land to have Port Town enter untapped.
4/8/2016 If an effect instructs you to put Port Town onto the battlefield tapped, it will still enter the battlefield tapped even if you reveal a land card from your hand.