There are two versions of Mx in this subdirectory:

One for aix 3.2 that produces text output (mx_aix3.2.Z)
One for aix 3.2 that produces html output (mx_aixhtml.Z)

You should grab the one for the operating system on your rs6000ish
system.  They are both stored in compressed format, so make sure you
transfer using binary ftp, and then

uncompress mx_aix3.2.Z
mv mx_aix3.2 mx

should give you the binary executable file mx.  If by default files ftp'd to
your machine are not executable then you will need to do

chmod +x mx

to make it executable - put it into /usr/local/bin or somewhere 
convenient that is in the user's path.  It can be run using

mx < inputfile > outputfile

See the directory ~ftp/pub/mx/examples for example files.

See the README file in the ~ftp/pub/mx directory for more details on versions,
documentation and examples.