I got this on my laptop:
Progress state: NS_ERROR_FAILURE
VBoxManage: error: Failed to install "/home/klu/tmp/Oracle_VM_VirtualBox_Extension_Pack-4.2.18-88780.vbox-extpack"
VBoxManage: error: The installer failed with exit code 127: Error creating textual authentication agent: Error opening current controlling terminal for the process (`/dev/tty'): No such device or address
VBoxManage: error: Details: code NS_ERROR_FAILURE (0x80004005), component ExtPackManager, interface IExtPackManager
VBoxManage: error: Context: "int handleExtPack(HandlerArg*)" at line 1126 of file VBoxManageMisc.cpp

Fix via:
sudo vboxmanage extpack install tmp/Oracle_VM_VirtualBox_Extension_Pack-4.2.18-88780.vbox-extpack

Notice the sudo use.

By karlo