Missed RMD on inherited IRA, how to complete Form 5329 in TT to show the amount that should have been distributed?

If you haven't taken the RMD then take it as soon as possible.

You can request a waiver of the penalty if you have reasonable cause. Please see Relief for Reasonable Cause for additional information.

If you do not have a Form 1099-R from an inherited IRA or are 73 to trigger the RMD questions then you will have to use "forms" mode to file Form 5329 which is only available with the TurboTax Desktop software:

1. Enter the forms mode and click on "Open Form" and type "5329".
2. Scroll down to Part IX line 52a and enter the RMD amount that should have been taken. On line 53a enter the amount of the RMD that was actually taken (probably zero if it was missed).
3. Then in the box right under line 53 "Waiver of Tax Smart Worksheet" enter the same amount as line 52 (the RMD amount) under A. 
4. Click the "Explanation" button and enter the reason for missing the RMD and your statement requesting a waiver.