If you are talking about 'myrouters.txt' then I assume you are using the Open Innovations bundle install under windows.
With this package, the myrouters.txt will be used by some script to generate .cfg files for the devices listed, but only for standard interfaces.
If you want to monitor anything else special, or have special directives to add to these files, then you will need to create and modify them by hand. Read the MRTG documentation on
http://www.mrtg.org and it will tell you how to create a .cfg file.
You may need a number of special directives if the OIDs you want to monitor are not counters of network traffic measured in bytes.