In order for the Application Center to work when behind a firewall you need to configure the service endpoint so that the console can call the rest services when the console is open from outside the firewall.
In particular the JNDI property ibm.appcenter.services.endpoint must be set as the external address and context root of the applicationcenter.war web application.
You can look at https://pic.dhe.ibm.com/infocenter/wrklight/v6r1m0/topic/com.ibm.worklight.installconfig.doc/appcenter/r_ac_appres_endpoint.html for details on this configuration.
Hope this helps. Emmanuel