If you run TeamViewer for support and you are unable to get the TeamViewer Host to connect even if you enter the proxy details you need to do the following:

Add TCP port 5938 to the ISA.

I ran the Internet Connection Wizard (ICW and added two rules to the Firewall:

TCP port 5938 In
TCP port 5938 Out

This allowed access to the server.

To allow access to workstations on the network you need to open the ISA Server management and locate the rules you created in the ICW and add the internal network to each rule where it currently only lists local host.  The rules will have the following configured:

TCP port 5938 In   | From: External    | To: Local Host
TCP port 5938 Out | From: Local Host | To: External

Right click on TCP port 5938 In -> select Properties -> click on the To tab and add the network Internal.

Right click on TCP port 5938 Out -> select Properties -> click on the From tab and add the network Internal.

Your ports should be configured as:

TCP port 5938 In   | From: External                 | To: Local Host, Internal
TCP port 5938 Out | From: Local Host, Internal | To: External

Don't forget to click on the Apply button in the ISA management console to apply your changes.  You should now be able to connect to workstations on the local network as well as the server.