Office 2016 down to 2010 offer native 64-bit versions. XLS Padlock is an add-in for Excel, so it must match the same bitness as Excel.
For instance, for Office 2010: start Excel. Open the File menu, and then select Help.
On the right side, you can see information about your version of Office. It will show the precise version number and bitness.
If you're running the 32 bit version, you’ll need to download the 32-bit or x86 version of XLS Padlock.
Or, if you have the 64 bit version of Excel installed, you’ll see the 64-bit tag after your Office version number: get the 64-bit version of XLS Padlock.