How do I enter a K-1 from an estate, knowing the amount and knowing no tax was withheld, without having the actual K-1 yet? Or is it better to just file an extension?

You may or may not be able to enter the basic information from your IRS form Schedule K-1.  If so, in TurboTax Premium Online, you would follow these directions.

• Down the left side of the screen, click on Federal.
• Down the left side of the screen, click on Wages & Income.
• Click the down arrow to the right of S-corps, Partnerships, and Trusts.
• Click to the right of Schedule K-1.
• At the screen Tell us about your Schedule K-1, you tell the software which Schedule K-1 you received.
• For several screens, you will be entering data right off the K-1 form.

Go here to file a free Federal tax extension.