How do I get the MAC address of a network card using Delphi?

Get the JEDI conversion of the Microsoft IP Helper Library from the Project JEDI API Header Library - the file is IPHlpAPI.zip. Unpack the files, you need IpTypes.pas and IpHlpApi.pas. Then you can use something like this:

procedure TForm1.Button1Click(Sender: TObject);
var
  NumInterfaces: Cardinal;
  AdapterInfo: array of TIpAdapterInfo;
  OutBufLen: ULONG;
  i: integer;
begin
  GetNumberOfInterfaces(NumInterfaces);
  SetLength(AdapterInfo, NumInterfaces);
  OutBufLen := NumInterfaces * SizeOf(TIpAdapterInfo);
  GetAdaptersInfo(@AdapterInfo[0], OutBufLen);

  Memo1.Lines.Clear;
  for i := 0 to NumInterfaces - 1 do begin
    Memo1.Lines.Add(Format('%.2x:%.2x:%.2x:%.2x:%.2x:%.2x',
      [AdapterInfo[i].Address[0], AdapterInfo[i].Address[1],
       AdapterInfo[i].Address[2], AdapterInfo[i].Address[3],
       AdapterInfo[i].Address[4], AdapterInfo[i].Address[5]]));
  end;
end;

(All error handling omitted, you should add it of course.)

The GetAdaptersAddresses function is the preferred way to obtain adapters information since 2001 with Windows XP.

The adapter's information is returned in the IP_ADAPTER_ADDRESSES structure by the AdapterAddresses parameter.

The GetAdaptersAddresses function can retrieve information for IPv4 and IPv6 addresses.

The recommended method of calling the GetAdaptersAddresses function is to pre-allocate a 15KB working buffer pointed to by the AdapterAddresses parameter. On typical computers, this dramatically reduces the chances that the GetAdaptersAddresses function returns ERROR_BUFFER_OVERFLOW, which would require calling GetAdaptersAddresses function multiple times.

---

procedure TForm1.Button1Click(Sender: TObject);
const
  AF_UNSPEC = 0;
  GAA_FLAG_INCLUDE_ALL_INTERFACES = $100;
  WORKING_BUFFER_SIZE = 15000;
  MAX_TRIES = 3;
var
  pAddresses,
  pCurrAddresses: PIpAdapterAddresses;
  dwRetVal,
  outBufLen: Cardinal;
  i: Integer;
  macAddress: string;
begin
  Memo1.Lines.Clear;

  outBufLen := WORKING_BUFFER_SIZE;
  pAddresses := nil;
  i := 0;
  repeat
    if Assigned(pAddresses) then
      FreeMem(pAddresses);

    GetMem(pAddresses, outBufLen);
    if not Assigned(pAddresses) then
      raise Exception.Create('Memory allocation failed for IP_ADAPTER_ADDRESSES struct');

    dwRetVal := GetAdaptersAddresses(AF_UNSPEC, GAA_FLAG_INCLUDE_ALL_INTERFACES, nil, pAddresses, @outBufLen);
    Inc(i);
  until (dwRetVal <> ERROR_BUFFER_OVERFLOW) or (i = MAX_TRIES);

  try
    if NO_ERROR <> dwRetVal then begin
      if ERROR_NO_DATA = dwRetVal then begin
        MessageDlg('No addresses were found for the requested parameters', mtInformation, [mbOK], 0);
        Exit;
      end
      else
        raise Exception.Create(SysErrorMessage(dwRetVal));
    end;

    pCurrAddresses := pAddresses;
    while Assigned(pCurrAddresses) do begin
      if pCurrAddresses^.PhysicalAddressLength > 0 then begin
        Memo1.Lines.Add(pCurrAddresses^.FriendlyName);
        macAddress := '';
        for i := 0 to pCurrAddresses^.PhysicalAddressLength - 1 do begin
          if i > 0 then
            macAddress := macAddress + ':';
          macAddress := macAddress + Format('%.2X', [pCurrAddresses^.PhysicalAddress[i]]);
        end;
        Memo1.Lines.Add(macAddress);
        Memo1.Lines.Add('');
      end;
      pCurrAddresses := pCurrAddresses^.Next;
    end;

  finally
    if Assigned(pAddresses) then
      FreeMem(pAddresses);
  end;
end;