You can't at the moment as it's only intended to be something turned on temporarily in specific situations. (I guess you could make a command which opens the window and then turns checkbox mode on, but I'm not sure how well it would work. It might be easy or it might require some experimentation...)
Note that checkbox mode in Opus is quite different to the similar-looking mode added later in Explorer. If you just want an easy toggle-select mode, checkbox mode isn't really for that; instead, either use Power Mode and configure the mouse buttons however you want or, in any other mode, use the middle mouse button to toggle files on & off without affecting other files.
[quote]2) How to view RECYCLE BIN in folder view when I start DO?
I start DO via taskbar using a shortcut:
"C:\Program Files\GPSoftware\Directory Opus\dopus.exe" "C:\My Documents" (to always position itself in My Documents when I start DO)[/quote]
Change it to run this:
"C:\Program Files\GPSoftware\Directory Opus\dopusrt.exe" /open /trash
Note that this runs dopusrt.exe, rather than dopus.exe.
(PS: Please ask one question per thread.)