Installing x64:

Unzip each of the two zip archives into its own folder so you can tell them apart.

Install ScanPack x64 by running the associated setup.exe program (instead of the MSI).
This will install 64-bit ScanPack DLLs into C:\Program Files\IPGP\ScanPack\.
Then install the x64 SDK by running the associated setup.exe program (not the MSI).
This will install headers and import libraries into subfolders 'include' and 'lib'.
The 'examples' subfolder will contain a Demo C++ application that can be built for
either 32-bit or 64-bit platforms.  Unzip the archive anywhere you have write access.