This is it. Solved, done. I just could not read this correctly. Remove battery. Start flasher. Connect usb cable. Insert battery. (Now it is seen by kernel and dmesg shows it) I just didn't understand that it makes difference which order usb is connected and battery is inserted.
And then repeat until success. For me it took seven tries. Now I fully believe N900 is unbrickable (until I found method to brick it ).