The following is a copy of the keygen readme.txt.

On a linux server, keygen should be installed in /usr/local/bin/keygen
It needs to be here because the php code in the ubercart conditional action custom php code
for the 'Run Keygen' predicate contains the line:
 exec("cd /usr/local/bin/keygen/bin && ./keygen.sh ".$eargs, $output, $return);

Having installed it (by simply creating the /usr/local/bin/keygen directory and unzipping
the contents of keygen.zip into it) you can then do:

 cd /usr/local/bin/keygen/bin
 chmod 755 *.sh               [to set the scripts as executable]
 
then  
 ./keygen.sh --help

to see what to do.

You should then use the --server option to set the various SMTP parameters (see --help output).

This will create ~/.openvpms/keygen.properties

This then must be moved to the appropriate directory - it was created in your home directory - but
we need to put it the home directory of the user who is running apache - who is www-data - and
his home is /var/www

So we need to cd to /var/www and then mkdir .openvpms and copy keygen.properties into this and then
adjust the ownerships to www-data:www-data

If you need to play with keygen on a windows machine, you can unzip the package to any convenient
folder and then cd to its bin folder and then use keygen.bat