Craig writes:
> Attempting download from Enhydra website of the .zip file. MPE/iX 6.5
> responds with error:
> $unzip enhydra3.1.zip
> Archive:  enhydra3.1.zip
>   End-of-central-directory signature not found.[...]

I would suggest trying the gzipped tar version of the archive:

http://enhydra.enhydra.org/software/downloads/enhydra/3.1/enhydra3.1.tar.gz

Then do:

$ gunzip enhydra3.1.tar.gz
$ tar xvf enhydra3.1.tar

G.