AWS_KEY- Your AWS Access Key IDAWS_SECRET- Your AWS Secret Access KeyMC_EC2_INSTANCE_ID- Instance ID of the EC2 instance which is running MinecraftMCPANEL_AUTH_PASSWORD- Password to allow access to the web interface
Once you have MCPanel running on Heroku, point your browser to the URL (e.g. mymcpanel.heroku.com) and it will prompt you to log in.
The password is what you specified as the MCPANEL_AUTH_PASSWORD environment variable.
Currently the interface is very sparse, only providing the current status of the EC2 instance and, if it is "stopped", a button to start the instance.
In fact, at this point there are no Minecraft-specific features and could be used for any generic EC2 instance.