How to obtain mouse cursor coordinates from lParam from a low level mouse callback method?

According to codeguru forum and especially pinvoke.net you are looking probably for (pinvoke.net again):

[StructLayout(LayoutKind.Sequential)]
 public struct MSLLHOOKSTRUCT
 {
     public POINT pt;
     public int mouseData; // be careful, this must be ints, not uints (was wrong before I changed it...). regards, cmew.
     public int flags;
     public int time;
     public UIntPtr dwExtraInfo;
 }

Then of course, you could always get current coordinates. Lots of this here on Stackoverflow.