George,
First of all, get rid of the firewall/router because the sme box is a firewall/router and now you have 2 in your system.
Connect the external nic from the sme box directly to your dsl/cable modem and reboot everything to establish new IP addresses (this assumes that you configured the sme box and a gateway/firewall and picked the right type of internet connection). Go to your console and initiate a "Check Internet Connection".
If you have a connection, you can then start to connect the lan and get your client machines working.
If you have no connection, report back with your settings, modem type, internet connection type, etc and we'll see if we can help.
Another great resource is the online manuals at
http://edocs.mitel.com It has pretty much step-by-step instructions for you there.
Good Luck,
Paul