.dec File Format
Decked Builder saves decks in the .dec file format, which is a popular file format for exchanging decks over the Internet. For example, Magic Online allows you to export decks in .dec format.
The .dec file format is a text file format, so you can open them in editors like Notepad on Windows or TextMate on the Mac. An example .dec file is shown below:
// A sample .dec file
4 Arid Mesa
4 Marsh Flats
5 Mountain
5 Plains
4 Scalding Tarn
3 Teetering Peaks
4 Goblin Guide
4 Kor Skyfisher
4 Plated Geopede
3 Spikeshot Elder
4 Steppe Lynx
3 Stoneforge Mystic
3 Adventuring Gear
4 Forked Bolt
3 Journey to Nowhere
2 Koth of the Hammer
1 Sword of Body and Mind
SB: 1 Basilisk Collar
SB: 4 Cunning Sparkmage
SB: 3 Mark of Mutiny
SB: 4 Refraction Trap
SB: 3 Revoke Existence
If you paste the above text into a text editor and save the file as a .dec file, you will be able to email yourself the file and import it into Decked Builder. Make sure you are not using an editor like Word or Wordpad, as they save a bunch of extra info about formatting that the app can't read, and it will lead to errors. You can also drop the file into the Decked Builder app on iTunes to directly import it when your device is connected.
Basically, each line of the file consists of the number of cards and the name of card in the deck. A line that starts with SB: refers to a sideboard card. A line that starts with // is a comment line, and is imported as a note. The file format is really quite self explanatory from the example above, and is designed to allow you to easily copy and paste decks into the .dec file format.
.coll File Format
Decked Builder extends the .dec file format to collections by merely requiring that you put the name of the expansion after the card name surrounded by square brackets. The reason for this is because while decks do not really care which expansion a card is from, collections do. See below for an example of a .coll file. Collection notes are marked with // after the card name.
4 Fresh Meat [New Phyrexia]
2 Lost Leonin [New Phyrexia]
4 Bludgeon Brawl [New Phyrexia]
3 Thundering Tanadon [New Phyrexia]
2 Artillerize [New Phyrexia]
1 Psychic Barrier [New Phyrexia]
// collection notes for Psychic Barrier here
You can paste the text above into a text editor, save it as a .coll file, and import it into Decked Builder by uploading it from the page here:
http://www.mtgo-stats.com/convert_coll/em
As an alternative, a properly formatted CSV file would work as well (see the link above for more details).
You can upload these at http://csv.deckedbuilder.com where a sample of a .csv file format compatible with Decked Builder is provided.