Virtual Service not on available network

Hi, I need to publish Outlook Anywhere through Kemp Load Master v7.0.6. When I try to add a new virtual service, it pops up a message "Virtual Service not on available network". The Kemp virtual device is in a DMZ. It has 2 NICs, 1 NIC is in the network A, the other NIC is in the network B, and the Exchange 2010 CAS server is in the network C. The firewall is configured to allow TCP 443 and ICMP-ping from the Kemp device to the CAS server. I'm able to ping CAS, but cannot create a new virtual service. Do you know if I need to configure anything else for this to work?

Thanks

Zoran