By just running below command it worked
sudo chown -R bitnami:daemon /opt/bitnami/apps/wordpress/your_plugin_directory
sudo chmod -R g+w /opt/bitnami/apps/wordpress/your_plugin_directory
Taken from here: http://wiki.bitnami.com/BitNami_Cloud_Hosting/Servers/Access_your_machine#Files_writable_by_the_web_server