If case of compilation error when assigning:

VideoGrabber->OnFrameCaptureCompleted = VideoGrabberFrameCaptureCompleted;

replace the types below in bold:

void __fastcall TForm1::VideoGrabberFrameCaptureCompleted(TObject *Sender,
Pointer FrameBitmap, int BitmapWidth, int BitmapHeight, DWORD FrameNumber,
__int64 FrameTime, TFrameCaptureDest DestType, UnicodeString FileName,
bool Success, int FrameId)
{
   ...
}

by:

void __fastcall TForm1::VideoGrabberFrameCaptureCompleted(TObject* Sender,
void* FrameBitmap, int BitmapWidth, int BitmapHeight, unsigned FrameNumber,
__int64 FrameTime, TFrameCaptureDest DestType, System::UnicodeString FileName,
bool Success, int FrameId)
{
   TBitmap *pFrameBitmap = (TBitmap*) FrameBitmap;
   ... do what you need with pFrameBitmap->...
}