You will need to determine the IPAddress which AWS lambda uses to call your instance. After you have gotten your IPAddress you need to allow it to access your chain..
You will need to add a line to your multichain.conf:
rpcallowip=<ipaddress>
This will allow AWS to access your chain. Also check that you have the relevant firewall permissions set to allow calls to be made to your instance on the ports set in your param.dat when you setup the chain.