The reason for that seems to be a PAUSE indexer problem. CPANPLUS (just like CPAN.pm) uses the 02packages file to determine what files can be gotten from the CPAN mirrors. Grepping that file for PDL::NetCDF yields no results:
$ zgrep PDL::NetCDF 02packages.details.txt.gz
That probably means something went wrong while indexing it, or the author used a very non standard distribution.
Luckily, CPANPLUS has support for installing from a URL as well, so you can still install it using:
$ cpanp -i http://search.cpan.org/CPAN/authors/id/D/DH/DHUNT/PDL-NetCDF-0.93.tar.gz
If you're interested in getting to the root of the problem, you might want to get in touch with the PAUSE admins.
Their contact details can be found here: