Since Debits and Credits appear in the same field, you have to separate them. You'll have to make a calculated field for both Debits and Credits that look something like this:
Debits: IF {DC} = "D" THEN {Amount} else 0
Credits: IF {DC} = "C" THEN {Amount} else 0
(My syntax is always wrong since I switch back and forth between MSAccess and Crystal - use the Builder to confirm your syntax)
Once you have Debits and Credits separated, you can summarize them and use them in other calculations.