Radio button is it checked?

I finally figured out how to set a radio button in my dialog to my 'default' state that I want.

Now, I want to check to see if the user selects my other radio button and when he or she does to change the value of a variable to a negative number. When I tried using BM_GETCHECKED my compiler chokes and says it is an undeclared identifier.

What is the correct way to implement this?
I've rem'd out the various things that I have tried below.

Here is the code I tried...

Code:

BOOL CALLBACK CircleProc(HWND hwnd, UINT Message, WPARAM wParam, LPARAM lParam)
{
	int nLen = 0;
	LPTSTR pbuffer = NULL;

    switch(Message)
    {
        case WM_INITDIALOG:
			// Default radio button.
			hdlg = GetDlgItem(hwnd,IDC_RADIO_ADD );
			SendMessage(hdlg, BM_SETCHECK, 1,0);

        return TRUE;
        case WM_COMMAND:
            switch(LOWORD(wParam))
            {
                case IDOK:
						{
						hdlg = GetDlgItem(hwnd, IDC_EDIT_DIAMETER);  //(hOwner, hDialogEditField)
						nLen = GetWindowTextLength(hdlg);
						pbuffer = (LPTSTR)malloc(nLen + 1);

						GetDlgItemText(hwnd,	//(hOwner, hDialogEditField)
							IDC_EDIT_DIAMETER,	// control identifier
							pbuffer,			// pointer to buffer for text
							nLen+1				// maximum size of string + NULL
						);

						// pbuffer is valid so do calculations.
						diameter = strtod(pbuffer, NULL);
						free(pbuffer);

						Ixc = (PI/64)*(pow(diameter, 4.0));
						area = PI * pow(diameter/2, 2);

// Here are the varoius things that I have tried.
					//	int iCheck;
					//	iCheck =  SendMessage (IDC_RADIO_SUBTRACT, BM_GETCHECKED, 0, 0);
					//if(Button_GetCheck(hwnd,IDC_RADIO_ADD ))
					//	if (iCheck != 0)
					//		area = -1*area;
						}
						// Since OK call center coord. dialog.
						DialogBox(GetModuleHandle(NULL), MAKEINTRESOURCE(ID_CENTER_COORDINATES), 
							hwnd, CenterCoordinates);
						EndDialog(hwnd, IDOK);
                break;
                case IDCANCEL:
					MessageBeep (0) ;
                    EndDialog(hwnd, IDCANCEL);
                break;
            }
        break;
        default:
            return FALSE;
    }
    return TRUE;
}

BM_GETCHECK and BM_SETCHECK

Code:

	if(SendMessage(GetDlgItem(hWnd,IDC_CHECK), BM_GETCHECK, (WPARAM)0 ,(LPARAM)0) == BST_CHECKED)

You are AWESOME novacain. Thanks for the response.