I usually use one field for "debits" and one for "credits" when setting up a bookkeeping ledger type layout: https://dl.dropboxusercontent.com/u/78737945/AccountingLedger.fmp12
But you can set up two calculation fields:
If (TransactionType = "Expense" ; Amount )
would return a value only for Expense amounts. A summary field can then summarize this calculation field.
If ( TransactionType = "Income" ; Amount )
could be used for income transactions in the same manner.
Thank you. It worked and I was able to figure out the summary field calculations, too. I appreciate your help.