The way it works is you need to know the number of bitmaps on your bmp image (it's the same thing as mIRC's button bmp strip)

You must load your bitmap file into a bitmap handle to pass to the function.

nImg = N; <-- number of icons on the bitmap
HBITMAP hbmp = LoadBitmap(Other parms);
tbadd.hInst = NULL;
tbadd.nID = hbmp;

SendMessage(hwnd, TB_ADDBITMAP, (WPARAM) nImg, (LPARAM) &tbadd);

Hope you get it working =], it's a nice feature to add to mIRC's toolbar already, but most people tend to replace mIRC's toolbar for more functionnality eheh.

Good Luck