Installing Propel behaviours with Composer
This is an autoloading issue.
Please check that you have
propel.behavior.equal_nest.class = vendor.craftyshadow.propel-equalnest-behavior.src.EqualNestBehaviorin your build.properties (for Propel).
Please check that the composer generated autoloader file is included during the bootstrap process of your application. Composer generates a "vendor/autoload.php" file. If you include it, then you get autoloading for free. And everything installed by Composer is found automatically.
require 'vendor/autoload.php';