The Mercadian Masques set doesn't add any new rules to the Magic: The Gathering environment. However, some of the card abilities may be a little confusing to players who haven't encountered them before. Here's a quick guide to those abilities.
Alternative Play Costs
Some card give you the option to pay an alternative cost to play the card instead of paying its normal mana cost. For example, Ramosian Rally says "If you control a plains, you may tap an untapped creature you control instead of paying Ramosian Rally's mana cost." These substitute costs are paid at the same time you would pay the normal costs. If another card requires you to pay additional mana to play a spell, you still have to pay that extra amount even if you use the alternative costs. Cards that check the mana cost of a spell, like Spell Blast, still check the normal cost, regardless of how you play the spell.
High Seas ("Red creature spells and green creature spell cost more to play") is in play. You play Rushwood Legate ("If an opponent controls an island and you control a forest, you may play Rushwood Legate without paying its mana cost"). You control a forest and your opponent controls an island, but you still have to pay to play the Legate, because of the High Seas.
Your opponent could then play Spell Blast for to counter your Legate. The X in Spell Blast's cost is 3 because the normal mana cost of the Legate is - it doesn't matter whether you played the Legate by using its alternative play cost.
Additional Play Costs and Choices
The phrase "As an additional cost to play *card name*" indicates an extra cost to play a spell. The extra cost must be paid when you play the spell, just like the normal cost. If you can't pay the additional cost, you can't play the spell. If the spell is countered, you'll have already paid the additional cost.
The phrase "As you play *card name*" indicates choices made when you play the spell. The phrase "As *card name* comes into play" indicates choices or costs that are requried as part of putting a permanent into play. These apply regardless of how the permanent is put into play; it doesn't matter whether it's created directly by a spell or brought into play by another effect.
A few cards have abilities that may be played by any player, not just by the card's controller. (This matters only if another card bases its effects on the controller of an ability. For example, some abilities trigger if a spell or ability controlled by another player forces you to discard.)
Example: Your opponent has Scandalmonger (": Target player discards a card from his or her hand. Any player may play this ability but only if he or she could play a sorcery.") and Spiritual Focus ("Whenever a spell or ability an opponent controls causes you to discard a card, you gain 2 life and you may draw a card.") in play. If you play the Scandalmonger's ability to make your opponent discard, you opponent will draw a card and gain 2 life. This is because even though your opponent controls the Scandalmonger card, you control the ability you played.
"Search Your Library"
Some cards let you search your library or your opponent's library for cards meeting some requirement and then do something with them. If you don't find what you're searching for, you do nothing. You're not required to search thoroughly; in fact, you may choose not to find what you're searching for, even if it's there.