How do I connect the common wire in a Carrier air handler?
There is a set of connections in the air handler that usually includes the "C" terminal, and it should be labelled (though it may be hard to read depending on the location of the control board and/or wiring/etc in the way).

One way to tell for sure is that while the furnace is not running, using a multi-meter you can measure between the "C" and "R" (or Rh or Rc terminals) and you should see 24VAC. R (or Rh and Rc) will already be going to the thermostat. 

As an example, on your furnace, from the manual:



The colors may match, but don't count on it. The sure-fire way is to check using a multi-meter as I said above.