Resolution
Parallels Panel is supplied with two MySQL services. The the first service is used for client databases, the second one is used to store Parallels Panel databases.1. The client's MySQL is installed to "%plesk_dir%\Databases\MySQL" folder by default. Its config file is located in "%plesk_dir%\Databases\MySQL\Data\my.ini" file.
You can verify the location opening "HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MySQL\Image" windows regestry key.
2. MySQL server for Parallels Panel is installed to "C:\Program Files\Parallels\Plesk\MySQL\" folder, where config is "C:\Program Files\Parallels\Plesk\MySQL\Data\my.ini" file.
Path to executable of MySQL server for Parallels Panel is stored in "MYSQL_VAR_D" key of this windows branch "KEY_LOCAL_MACHINE\SOFTWARE\PLESK\PSA Config\Config"
Note: Jet is used as Parallels Panel database engine by default and MySQL server is disabled.