The S corp is technically a calendar year. The extended due date of the 2015 1120S was September 15, 2016. This extended due date then gives the shareholder's one month to then file their individual tax return which was due October 15, 2016.
Since this is a 2015 K-1, this belongs on your 2015 1040. This means you will need to amend your 2015 1040.
This also means that you will then receive a 2016 K-1. If you do not receive this in time to prepare your 1040, then you have two options; (1) prepare your return and amend, or (2) extend your tax return and file it once it is received. I would recommend extending your personal return. In doing this, you need to estimate the impact that this K-1will have on your personal return and take that into account if you owe any $$ at April 18, 2017.
You will enter your K-1 information on Schedule E. Follow the input directions and you should be fine.
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