Answered my own question. Do the following:
Login as NSM (if you login as root, you will have to perform a chown on the dmi-schema-stage/nsm directory)
Change to this directory:
/usr/netscreen/GuiSvr/utils/subversion-1.4.4/bin
and perform the following commands:
./svn cleanup
./svn update /usr/netscreen/GuiSvr/var/dmi-schema-stage/nsm
Then you can perform your update via your Gui as normal.
Hope this helps someone!