usually you enhance your message map of the view where you want to mouse input


BEGIN_MESSAGE_MAP( CMyview, classtype your derive CMyview from)
//{{AFX_MSG_MAP( CMyview)
ON_WM_LBUTTONDOWN( )
//}}AFX_MSG_MAP
END_MESSAGE_MAP( )


and overload afx_msg void OnLButtonDown( UINT, CPoint ); in your view class


this way you get a message whenever you left clicked into the desired view

for additional information have a look at the msdn

google: msdn mfc


as side note:
before you dive too much into MFC id rather suggest you to have a look at wxwidgets

wxwidgets

its a cross platform gui library that is very similar to mfc except with one advantage, the code looks thousand times clear which is why I use wxwidgets in all my gui app projects

I am working on a level editor on my own

The way I do it is giving each window a id
the 3d camera view will use the gluProject unproject functions

the 2d views use the view ids to determine the picking ray


the final information of the ray is then passed to the scene manager that does the rest for me


i do all modifications with modifiers,

those are stored on a stack to support a undo system, pretty important in almost every editor

i simply store a copy of the unmodified object in the modifier instance and a link to the new modified object this way i can easily undo any changes

this might not be the most efficient way but it works and can be enhanced/improved later on