Index -> Interface -> Leagues -> League Deck Builder

Each League has its own individual League Deck Builder, reached by clicking on the Deck Builder Tab while in that League's League Room. This is a variant of the normal Deck Builder. Also, if you're playing in a Sealed League, this is where you can build cards from your limited card pool.

It is important to note that each individual League has it's own individual Deck Builder. If you are playing in Sealed Deck League A and Sealed Deck League B, you need to access the League Deck Builder from the League A League Room to build decks for use in League A and the League B League Room to build decks with cards from your League B card pool.

League Deck Builder in Sealed Play

In a Sealed Deck League you must use the League Deck Builder to make a valid deck for your first game. In a Sealed Deck League you may ONLY use cards that come from your starting card pool (in the initial Leagues, this consists of the Starter you brought) or those opened with the 'Add Booster' option (see Leagues and League Rooms for info on Adding Boosters). These cards are part of you collection as a whole (as in, anything that gets used in a Sealed League is yours for use outside of the League though it's important to note that while the Sealed League is running, you can't trade away any cards being used in that Sealed League) but for games in Sealed League play, you are considered to have ownership of the cards you see in the League Deck Builder and nothing else.
If you are playing in multiple Sealed Deck Leagues at the same time, be sure to remember which deck files go with which League since each Sealed Deck League will have its own card pool.

See also Deck Builder, Leagues, League Rooms, League Lobby, and Joining a League.