1)
-Change into you Plesk Admin htdocs directory (e.g. /opt/psa/admin/htdocs)
-Make a directory for the Greylisting Config Panel (e.g. "mkdir greylisting")
-Set chown of the directory you made above to root:psaadm ("chown root:psaadm greylisting")
-Set chmod of the directory you made above to 755 ("chmod 755 greylisting")
-Extract all files of the ZIP into that directory
-Set chown of all files to root:psaadm ("chown -R root:psaadm *")

-make your settings in file config.inc.php

2)
-Login to your Plesk as admin
-Make a new Custom Button with the following settings:

Button label: Just choose a name ;-)
Location: Domain administration page
Priority: 100
Background image: choose the gllogo.gif of the ZIP
URL: your directory you made above an the index file (e.g. "/greylisting/index.php")
Include domain id: yes
Include domain name: yes
Include FTP Login: no
Include FTP password: no
Include client id: yes
Include client's company name: no
Include client's contact name: no
Include client's e-mail: no
Context help tip contents: whatever you want
Open URL in the Control Panel: yes
Visible to all sub-logins: yes (if your clients are allowed to manager their greylisting)