What operating system are you using? If you want to test the script from say a unix shell then you can just type:

./subscribe.pl --send

at the command prompt - providing the permissions are -rwxr-xr-x (ie it is executable). The call to perl is in the first line of the script so providing it finds perl it will run (and since this is all set at install time this should be ok).

Now to cron it thats your next step.

As per the instructions just put:
"0 2 * * * /path_to/gforum/admin/cron/subscribe.pl --send" in your crontab.

If you have RedHat linux might just put a command file in /etc/cron.daily that just has:
/path_to/gforum/admin/cron/subscribe.pl --send
in it. Make sure the command file is rwx-r-xr-x as well. Take a look at other scripts in /etc/cron.daily (or hourly) to see how to do it.

doug