Here is a simple setup:
In the wireless router bridge the WLAN interface with the ethernet interface which is plugged into the main router. In Winbox (connect via the MAC address will creating the bridge):
> Click on the Bridge button and then the plus sign (a new window will pop-up), click ok and then click on the Ports tab. Click on the plus sign and add the WLAN interface (everything besides the interface remains at the default value), then click ok. Add the ethernet interface by clicking on the plus sign again and click ok.
Set an IP address on the bridge:
> Click on IP and then Addresses. Add the address of x.x.x.x/24 interface=bridge1
Set the Gateway
> Click on IP and then Routes. Click on the plus sign and type in the address of the main router in the "Gateway" field (Everything else remains the same.)
All that will be left is setting up the wireless interface, but I'n not sure what you needs are there. The Wiki has some examples and you can also check out some of the examples at
http://www.titansvcgroup.com/free_scripts.htm.