1. Export your deck and save it as a text file
(*.txt). Do not save it as a (*.dec) file.
2. Save your deck to your MTGO deck folder. On Windows
machines, this is usually located here: C:\Program Files\Wizards
of the Coast\Magic Online\decks.
3. To import this deck into MTGO, click on the "Load" button in
the Deck Editor. You will not see this deck immediately after
clicking "Load". You must select *.txt files instead of
*.dec files from the drop-down box.