Hi guys,
I have a CISCO 5500 Firewall and it already has 2 networks connected: one connecting to the internet and the other one connecting my private network.
I have now connected another network on one of the interfaces but I am unable to enable communication between the new network and the existing private network. I have tried following Cisco instructions on how to allow inter-interface traffic but so far I have been unsuccessful. Is it to do with NAT-ing or something? Kindly assist.

I was able to sort out this problem (albeit after much struggle :( ) by adding a dynamic translation(PAT) for traffic from my 'old' private network to an IP on the new network. I am now able to reach hosts on the new network from the 'old' network.
Hope this is of help to someone else.

Glad you figured it out,
was going to say you need to allow a dynamic any on the inside translated to the outside mark sure that all you networks have exempt nats as well