You cannot open a 2015 data file with 2016 TurboTax. The *.tax2015 data file can only be opened by 2015 desktop TurboTax (CD/download). 2016 TurboTax can only open a 2016 data file.
You can transfer the *.tax2015 data file into a new 2016 return, but you can't "open" the 2015 data file with 2016 product.
If you saved a PDF of your 2015 return, you should be able to open the PDF in Adobe Reader or other PDF viewer.
