Seems like I have a problem with this as well...
pkg install io -forge
warning: creating installation directory C:\Octave\Octave-4.0.0\share\octave\pac
warning: called from
install at line 30 column 5
pkg at line 405 column 9
'g++' is not recognized as an internal or external command,
operable program or batch file.
make: *** [csvexplode.oct] Error 1
make: Entering directory `/tmp/oct-4hBfay/io/src'
make: Leaving directory `/tmp/oct-4hBfay/io/src'
pkg: error running `make' for the io package.
error: called from 'configure_make' in file C:\Octave\Octave-4.0.0\share\octave\
4.0.0\m\pkg\private\configure_make.m near line 96, column 9