Not sure from your post if your exact current hardware configuration once worked, or if the problems started from day one of your current hardware setup.

Try turning off all security settings in the routers to first get things working.

The DHCP server should be on in no more than ONE router. You cant have two routers arguing over who will assign IP addresses. Conceivably, you could also turn off both DHCP servers and manually assign all your IPs and other configuration settings. I am unfamiliar with Airports but I am assuming that the DHCP server can be switched off which will allow it to still function as a "switch" (which is a "smart" hub) and as a WiFi gateway.

I think if you turn off the DHCP server in the Airport, you will still be able to connect through the Airport on the way to the ActionTec with an IP assigned by the ActionTec. It is easyer to use the DHCP server in the ActionTec because it, not the Airport is the gateway to the Internet. Also, your laptop wont have to be manually configured. If you were to use the DHCP server in the Airport, you would still have to do some manual settings in both the mac and the PC.

But if you must resort to manual settings, be sure that the Macs control panel settings:
lists the ActionTecs LAN IP, not the Airports IP as the Router,
Manual IP setting is within the ActionTecs range of IPs and is different from other IPs on your network
May also have to manually set DNS Servers in the control panel (just copy the two listed in the ActionTecs configuration or status page accessed through your browser.