How to add a C wire to an old Lennox system
In the photo that shows the insides of the furnace. On the left side of the photo there's a yellow and a red wire connected together. That should be where you want to connect the C wire.

you can confirm this by tracing the wires.  From the thermostat cable, the red wire should connect with the blue and orange wire.  While the blue wire connects to a white wire, with a blue twist-on wire connector. There's another blue twist-on wire connector where two red wires connect. One of the red wires is grouped with the white wire (in the same cable), those go to the A/C.  The other red wire should run over and connect to the yellow wire, using a yellow twist-on connector. 

If all this is true (it's difficult to trace the red wires through the photo), then the yellow wire from the thermostat cable (new C wire) should connect with the red and yellow wires.