1)
-Change into your Plesk Admin htdocs directory (e.g. /opt/psa/admin/htdocs)
-Make a directory for the Plesk FTP User Manager (e.g. "mkdir ftpuseredit")
-Set chown of this dir (e.g. "FtpUserEdit") to root:psaadm ("chown -R root:psaadm ftpuseredit")
-Extract all files of the ZIP into that directory
-Set chown of all files to root:psaadm ("chown -R root:psaadm *")
-Set chown of the file "wrapper" to root:root ("chown -R root:root wrapper")
-Set chmod of "vhost_writer.sh" to 0755
-Set chmod of "wrapper" to 4755
-edit the 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 logo.gif of the ZIP
URL: your directory you made above an the index file (e.g. "/ftpuseredit/index.php")
Include domain id: yes
Include domain name: yes
Include FTP Login: yes
Include FTP password: no
Include client id: no
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 FTP Accounts) ["no" in Plesk 10.x]

IF YOU'RE USING THE ADDONS WITH PLESK 10.x A SPECIAL CONTROLLER IS NEEDED!
DOWNLOAD THE CONTROLLER AND INSTALL WITH THE INSTRUCTIONS FROM THE INSTALL.txt

Download -> http://haggybear.de/download/psa10_controller.zip

Since Plesk 10.4.x no additional controller is needed!