Main Step is the fifth Step in an Attack Subturn. In this Step, a player in the Attack Subturn can summon a Seal, order a Seal to attack or use a Skill, spell a Mystic Card, and order a Mystic Card to use a Skill whether the card has an Interfere or not. On the other hand, a player in a Defense Subturn cannot do it (Except ordering or summoning/spelling a card that has an Interfere in Interfere Step).
If there is an Ability which effects a player to do something by not specify to do it in any specific Step, the player can do it only in a Main Step in an Attack Subturn.