Depends on where you saved the draft from. A button named "Load Draft" will appear next to the "Save draft" button when applicable. For example if you were replying to a topic and saved it as a draft, came back a few days later and went to reply to that very same topic you'll now a have a "load draft" button.
If you were starting a new topic and saved a draft, then came back a few days alter and went to start a new topic in the same forum you'll get the "Load Draft" button.
Lastly you can find them all from your control panel. If you click the control panel link on the top on the left side you'll find a link "Manage drafts"