Problem In Using Extension

Hi
i m trying to use the extension EchMultiSelect But on my form I am getting following error :
Alias "ext.EchMultiselect.EchMultiselect" is invalid. Make sure it points to an existing PHP file and the file is readable."

I also try to add following lines in my config/main.php

'import'=>array(
		'application.models.*',
		'application.components.*',
		'application.extensions.*',
	),

even then im getting same message, kindly guide