In every OS, the NI-VISA library bitness (i.e. 32- or 64-bit) has to match the Python bitness. So first you need to install a NI-VISA that works with your OS and then choose the Python version matching the installed NI-VISA bitness.
PyVISA includes a debugging command to help you troubleshoot this (and other things):
According to National Instruments, NI VISA 17.5 is available for the following platforms.
If NI-VISA is not available for your system, take a look at the Frequently asked questions.
Mac OS X
Download NI-VISA for Mac OS X
Mac OS X 10.7.x x86 and x86-64
Mac OS X 10.8.x
64-bit VISA applications are supported for a limited set of instrumentation buses. The supported buses are ENET-Serial, USB, and TCPIP. Logging VISA operations in NI I/O Trace from 64-bit VISA applications is not supported.
Download NI-VISA for Windows
Windows Server 2003 R2 (32-bit version only)
Windows Server 2008 R2 (64-bit version only)
Windows 8 x64 Edition (64-bit version)
Windows 8 (32-bit version)
Windows 7 x64 Edition (64-bit version)
Windows 7 (32-bit version)
Windows Vista x64 Edition (64-bit version)
Windows Vista (32-bit version)
Windows XP Service Pack 3
Support for Windows Server 2003 R2 may require disabling physical address extensions (PAE).
Download NI-VISA for Linux
Red Hat Enterprise Linux Desktop + Workstation 6
Red Hat Enterprise Linux Desktop + Workstation 5
Scientific Linux 6.x
Scientific Linux 5.x
More details details can be found in the README of the installer.
NI-VISA runs on other linux distros but the installation is more cumbersome. On Arch linux and related distributions, the AUR package ni-visa (early development) is known to work for the USB and TCPIP interfaces. Please note that you should restart after the installation for things to work properly.