how to get the MAC address of the network card

You should familiarise yourself with the Terminal application on Mac OS X and the neat utility 'man'. I got the following result when I did a search in the man pages useing 'man':

[myhost:~] orangehead911% man ethers
ETHERS(3)               System Library Functions Manual              ETHERS(3)

NAME
     ethers, ether_line, ether_aton, ether_ntoa, ether_ntohost, ether_hostton
     - Ethernet address conversion and lookup routines

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <sys/types.h>
     #include <sys/socket.h>
     #include <net/ethernet.h>

     int
     ether_line(const char *l, struct ether_addr *e, char *hostname);

     struct ether_addr *
     ether_aton(const char *a);

     char *
     ether_ntoa(const struct ether_addr *n);

     int
     ether_ntohost(char *hostname, const struct ether_addr *e);

     int
     ether_hostton(const char *hostname, struct ether_addr *e);

DESCRIPTION
     These functions operate on ethernet addresses using an ether_addr struc-
     ture, which is defined in the header file <netinet/if_ether.h>:

           /*
            * The number of bytes in an ethernet (MAC) address.
            */
           #define ETHER_ADDR_LEN          6
           /*
            * Structure of a 48-bit Ethernet address.
            */
           struct  ether_addr {
                   u_char octet[ETHER_ADDR_LEN];
           };

     The function ether_line() scans l, an ASCII string in ethers(5) format
     and sets e to the ethernet address specified in the string and h to the
     hostname.  This function is used to parse lines from /etc/ethers into
     their component parts.

     The ether_aton() function converts an ASCII representation of an ethernet
     address into an ether_addr structure.  Likewise, ether_ntoa() converts an
     ethernet address specified as an ether_addr structure into an ASCII
     string.

     The ether_ntohost() and ether_hostton() functions map ethernet addresses
     to their corresponding hostnames as specified in the /etc/ethers
     database.  ether_ntohost() converts from ethernet address to hostname,
     and ether_hostton() converts from hostname to ethernet address.

RETURN VALUES
     ether_line() returns zero on success and non-zero if it was unable to
     parse any part of the supplied line l.  It returns the extracted ethernet
     address in the supplied ether_addr structure e and the hostname in the
     supplied string h.

     On success, ether_ntoa() returns a pointer to a string containing an
     ASCII representation of an ethernet address.  If it is unable to convert
     the supplied ether_addr structure, it returns a NULL pointer.  Likewise,
     ether_aton() returns a pointer to an ether_addr structure on success and
     a NULL pointer on failure.

     The ether_ntohost() and ether_hostton() functions both return zero on
     success or non-zero if they were unable to find a match in the
     /etc/ethers database.

NOTES
     The user must insure that the hostname strings passed to the
     ether_line(), ether_ntohost() and ether_hostton() functions are large
     enough to contain the returned hostnames.

NIS INTERACTION
     If the /etc/ethers contains a line with a single + in it, the
     ether_ntohost() and ether_hostton() functions will attempt to consult the
     NIS ethers.byname and ethers.byaddr maps in addition to the data in the
     /etc/ethers file.

SEE ALSO
     yp(4), ethers(5)

BUGS
     The ether_aton() and ether_ntoa() functions returns values that are
     stored in static memory areas which may be overwritten the next time they
     are called.

HISTORY
     This particular implementation of the ethers library functions were writ-
     ten for and first appeared in FreeBSD 2.1.

BSD                             April 12, 1995                             BSD