Code

Compiling 'test' for 'Arduino Uno'
Binary sketch size: 1176 bytes (of a 32256 byte maximum) (0.1430082 secs)
Compiling debug version of 'test' for 'Arduino Uno'
test.ino : In file included from
VM_DBG.h : ISO C++ forbids declaration of 'usb_serial_class' with no type
VM_DBG.h : expected ';' before '*' token
VM_DBG.h : 'usb_serial_class' has not been declared
VM_DBG.h : 'usb_serial_class' has not been declared
VM_DBG.cpp : In file included from
VM_DBG.h : ISO C++ forbids declaration of 'usb_serial_class' with no type
VM_DBG.h : expected ';' before '*' token
VM_DBG.h : 'usb_serial_class' has not been declared
VM_DBG.h : 'usb_serial_class' has not been declared
VM_DBG.cpp : variable or field 'begin' declared void
VM_DBG.cpp : 'usb_serial_class' was not declared in this scope
VM_DBG.cpp : 'theSerial' was not declared in this scope
Error compiling 

Code

int count;
void setup()
{
    count = 0;
    pinMode(13, OUTPUT);
}

void loop()
{
    digitalWrite(13, count%2);
    count ++;
    delay(500);
}