When you try to view/print/download your 2016 return, does it tell you that you need to upgrade to PLUS and pay to get access?  And have you already done the upgrading part?   I can tell you how to pay for your upgrade.

Also be aware that if a tax return transcript will suit your purpose, you may be able to download a free tax return transcript at this IRS website.  If you can't download it, then you could order it to come by postal mail, but that takes 5-10 calendar days.
http://www.irs.gov/Individuals/Get-Transcript

Before paying to unlock the 2016 return, make sure you have upgraded in the same account as the 2016 return--that you at least see evidence of the 2016 return in that same account when you look in the section "Your Returns & Documents."

Since you said you have already upgraded your 2017 return to PLUS, I will skip the upgrade steps and provide the payment steps.

• Log in and open the 2017 return you started and already upgraded "Take me to my return."
• Once the return is open and past the blue-green screen, click in the left menu column on Tax Tools, then choose Print Center.
• Then choose "Print/save/preview this year's return."  (Yes, you say "this year's" since you are paying for the 2017 return to unlock the prior returns.) You won't actually have to print it.
  
• On the next screen choose to "Pay now", and it should then prompt you for payment by credit or debit card.

NOTE:  If it should offer you the option to "pay later" out of the Federal refund, that can't be used for this unlocking purpose.  You will need to pay now by credit/debit card.