このようなもの:
int monthNumber = 11;
NSString * dateString = [NSString stringWithFormat: @"%d", monthNumber];
NSDateFormatter* dateFormatter = [[NSDateFormatter alloc] init];
[dateFormatter setDateFormat:@"MM"];
NSDate* myDate = [dateFormatter dateFromString:dateString];
[dateFormatter release];
NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
[formatter setDateFormat:@"MMMM"];
NSString *stringFromDate = [formatter stringFromDate:myDate];
[formatter release];
NSLog(@"%@", stringFromDate);
NSDateFormatterでmonthSymbols配列を使用してから、月番号のキーでその配列にアクセスできます。