First, if you prefer videos, check out the tutorial videos here.
To upload decks from your hard drive, first go to the “Collection” tab.
In the lower left area of the screen, you can import decks or start a new deck using an “Add Deck” button:
“Import” is one of the options in the next window:
Clicking “Import” will bring up a Windows file browser such as the one below. Find and select your decks on your hard drive (the default location for Windows 7 users will be C:\Users\[USERNAME]\AppData\Roaming\Wizards of the Coast\Magic Online\3.0\Decks).
Note that you can select as many decks as you like among .txt, .dek, and .csv formats for simultaneous import:
The client will make an educated guess about the format of each deck and save them into the appropriate format filters in the “Decks & Binders” section. Clicking on a Format will expand to show you the decks you have in it, and clicking on a deck will open it for viewing and editing.
If you want to change a deck’s format, you can open the deck and click the gear icon, or right-click on the deck and select “Properties.”
From the Properties, you can change a deck’s format through the drop-down menu:
Sometimes you have a deck in the correct format, but it is not legal to play in that format for some reason. You will see a yellow warning icon next to the deck name, and a “caution” treatment around the border of any problem cards. You can right-click on the warning icon or a problem card for a tooltip explaining the issue.
This happens most often when a format rotates, or the Banned & Restricted List is updated, and previously legal decks become illegal.
Note that if you put a version of a card into a deck that is not in your collection, but you have some version of it in your collection, the deck will be playable despite the warning icon. Such cards will be automatically replaced with versions from your collection when the match starts.