The version of phpmyadmin that ships with Plesk doesnt let you edit the user table. This is what ive had to do to allow this:
1) Download a full copy of phpmyadmin
HERE
2) uncompress it: tar zxvf phpMyAdmin-2.5.6.tar.gz
3) after its uncompressed rename the directory it just created, issue this command:
mv phpMyAdmin-2.5.6 myadmin
4) You will need to edit 2 lines in the "conf.inc.php" file located in the "myadmin" directory
open that file in an text editor (Vi or pico) and scroll down a little and look the following 2 lines:
$cfg['Servers'][$i]['auth_type'] = 'config';
$cfg['Servers'][$i]['user'] = 'root';
change them to look like:
$cfg['Servers'][$i]['auth_type'] = 'cookie';
$cfg['Servers'][$i]['user'] = '';
save and close the file.
5) Now you will need to move "myadmin" directory to a domain so you can access it from the browser:
mv myadmin /home/httpd/vhosts/MY_DOMAIN/httpdocs/
(replace "MY_DOMAIN" with a real domain you have on that server)
once its moved open and point you browser to that directory and log in with the same username/passwd combo that you use to log into plesk, you'll see a link called " Privileges ", that will list all mysql users, edit the user that need to access from another server, scroll down till you see the section "Change Login Information / Copy User", select "Any Host" from the combo box, the text field just next to it you should see it change from "localhost" to "%" , hit the go button to save the changes and that should do it.