Your definitions should be Uint32
:
private const UInt32 MOUSEEVENTF_LEFTDOWN = 0x02;
private const UInt32 MOUSEEVENTF_ABSOLUTE = 0x8000;
private const UInt32 MOUSEEVENTF_LEFTUP = 0x04;
private const UInt32 MOUSEEVENTF_RIGHTDOWN = 0x08;
private const UInt32 MOUSEEVENTF_RIGHTUP = 0x10;