In Mac OS X, I am getting an error in the log that states: "[ERROR]Failed to install tap/tun driver, kextload failed."

This error indicates  that Tunnelblick was unable to load the kexts (drivers) required to make OpenVPN work.
The most common cause is that there are already VPN kexts loaded which conflict with the BolehVPN-GUI kexts.

"Tunnelblick: openvpnstart status #247: Error: Unable to load net.tunnelblick.tun and/or net.tunnelblick.tap kexts in 5 tries. Status = 71"
"Tunnelblick: openvpnstart status #247: Error: Unable to load tun and tap kexts. Status = 71"

If you also see the above lines in your log please follow the steps detailed below:

1. Open a Terminal window. Type in kextstat
2. Scroll down the list of results. Look for something like foo.tun and foo.tap, com.cisco.cscotun, com.viscosityvpn.Viscosity.tun and com.viscosityvpn.Viscosity.tap or com.pogoplug.xcetun.
3. Type in sudo kextunload -b "name of tun or tap found in kextstat"
   (For example, if foo.tun or foo.tap are found, type in sudo kextunload -b foo.tun or sudo kextunload -b foo.tap)

This fix will likely be undone if you reboot the computer as the conflicting software will likely reload the kexts again.

The long term solution to this problem is to disable the conflicting program from launching at startup or uninstalling it.