If you are unable to open .SYLK flies; there are a few things to check to be able to do so.
Micosoft Excel
To be able to open .SYLK files, you will need access to Microsoft Excel. If you do require more information, take part in the Microsoft Office Suite Tutorial.
Malwarebytes
The once useful, Malwarebytes, will block Microsoft Excel from opening .SYLK Files so you will want to uninstall it from your device. There should be an option to whitelist the file but since Malwarebytes causes issues with other programs, you should just remove it and save yourself some headache.