For VIZ 2008 simply copy maxwell.dlr from Max9 \plugins directory to VIZ \plugins directory. Also, maxwell_mtlprop.ms and maxwell_camprop.ms from \scripts directory to your VIZ \scripts directory. For lower versions of VIZ, you have to use Max8/Max7 as the source.
Or you could let the installer do the job. When installing, put your VIZ 2008 path in the "Location for Max 9" edit field, and it will automatically do the job. If you have another VIZ version, try changing the other Locations.
In the next plugin update, VIZ 2008 will be supported from the installer.