If you aren't using generics the 'get' method is probably just returning an Object.
BudgetDatum budgetDatum; . . . budgetDatum = ((BudgetDatum) dataSource.get(rowIndex)).getDescription();
what I believe to be a syntax errorThere is too much here about what you believe, and not enough about what the compiler actually told you. If it was a syntax error the compiler would have said so, and if all your beliefs were correct you wouldn't be asking the question.
which leads me to believe I am accessing the member of dataSource incorrectly.