If you need to file an extension with the IRS, sign in to your account and click Account > 2013 Extension or Account > What Would You Like to Do > then choose the Begin your 2013 Extension.
Once you have started entering your extension information, you can navigate directly to it from Account on the menu bar, 2013 Extension on the menu and select the screen you need.
If you think you might not be able to file your tax return by the April 15th deadline, you should absolutely file an extension. That gives you protection against late filing penalties if you are unable to file by April 15th.
An extension, however, does not protect against interest or late payment penalties. You still need to pay the IRS any taxes owed by the April 15th deadline even if you file your tax return after that date. However, late filing penalties are a lot larger than interest or late payment penalties, so an extension should be filed to protect against those penalties. You can file a free extension on FREETAXUSA.
If you owe money to the IRS, you will need to send in your payment by April 15th in order to avoid interest or late payment penalties. However, even if you cannot pay the IRS the amount owed, you should either file your tax return or file an extension by April 15th in order to avoid late filing penalties. Late filing penalties are a lot larger than interest or late payment penalties. A Form 4868 extension, gives you an extra 6 months to file your tax return, but it does not give you an extra 6 months to pay any taxes owed to the IRS.