Plugin url : http://plugins.jetbrains.com/plugin?pr=&pluginId=7219
The plugin requires that you have the latest PHPStorm EAP build (6.0.1 EAP 129.196, Apr 5), available at http://confluence.jetbrains.com/display/PhpStorm/PhpStorm+Early+Access+Program
You can install the plugin by going to Settings -> Plugin -> Browse repositories and then search for Symfony2. RECOMMENDED WAY
You can also download the plugin .jar from github and then install it in phpstorm: Settings -> Plugins -> Install plugin from disk.
Open the project in IntelliJ, and follow the steps here : http://confluence.jetbrains.com/display/PhpStorm/Setting-up+environment+for+PhpStorm+plugin+development