Idaho doesn't have an extension form. They allow an automatic 6-month extension without a written request. I'm not sure if that "51 EXT" that you see is an old form number, but that would explain why you're not able to print it.
Here's the link to the 2016 instructions for Idaho Form 51, which is what you use to make a payment with your extension, if applicable: https://tax.idaho.gov/forms/EFO00092_04-18-2016.pdf.
Note the very first line of the instructions, under A. General Information, which explains the automatic 6-month extension.
