NSDecimalNumber *decimalNumber = ...;

NSNumberFormatter *formatter = [[NSNumberFormatter alloc] init];
[formatter setFormatterBehavior:NSNumberFormatterBehavior10_4];
[formatter setNumberStyle:NSNumberFormatterDecimalStyle];
[formatter setPositiveFormat:@"#,##0.00"];

NSLog(@"%@", [[formatter stringFromNumber:decimalNumber] description]);