Modify system_init.c.

A)
/*******************************************
 *  USB Device Descriptor 
 *******************************************/
const USB_DEVICE_DESCRIPTOR deviceDescriptor =
{
    0x12,                           // Size of this descriptor in bytes
    USB_DESCRIPTOR_DEVICE,          // DEVICE descriptor type
    0x0200,                         // USB Spec Release Number in BCD format
    USB_CDC_CLASS_CODE,         // Class Code
    USB_CDC_SUBCLASS_CODE,      // Subclass code
    0x00,                       // Protocol code
    USB_DEVICE_EP0_BUFFER_SIZE,     // Max packet size for EP0, see system_config.h
    0x04D8,                         // Vendor ID
    0x000A,                         // Product ID
    0x0100,                         // Device release number in BCD format
    0x01,                           // Manufacturer string index
    0x02,                           // Product string index
    0x03,                           // Device serial number string index  <-------- change from 0x00 to 0x03
    0x01                            // Number of possible configurations
};

B) add string descriptor for serial number
/*******************************************
 *  Serial number string descriptor
 *******************************************/
    const struct
    {
        uint8_t bLength;        // Size of this descriptor in bytes
        uint8_t bDscType;       // STRING descriptor type
        uint16_t string[4];    // String
    }
    sd003 =
    {
        sizeof(sd003),
        USB_DESCRIPTOR_STRING,
		{'y','t','s','1'}
    }; 

C) 
/***************************************
 * Array of string descriptors
 ***************************************/
USB_DEVICE_STRING_DESCRIPTORS_TABLE stringDescriptors[4]=	//<---------change 3 to 4
{
    (const uint8_t *const)&sd000,
    (const uint8_t *const)&sd001,
    (const uint8_t *const)&sd002,
    (const uint8_t *const)&sd003	//<--------- add this line
};

D) 
/*******************************************
 * USB Device Layer Master Descriptor Table 
 *******************************************/
const USB_DEVICE_MASTER_DESCRIPTOR usbMasterDescriptor =
{
    &deviceDescriptor,          /* Full speed descriptor */
    1,                          /* Total number of full speed configurations available */
    fullSpeedConfigDescSet,     /* Pointer to array of full speed configurations descriptors*/
    NULL, 
    0, 
    NULL, 
    4,                          //Total number of string descriptors available. <--------- change from 3 to 4
    stringDescriptors,          // Pointer to array of string descriptors.
    NULL, 
    NULL
};