I flashed my N900 again before christmas for the first time since 2013 and first time with this windows 7 PC.

I had a similar problem finding the N900 and had to install some addition driver so that Flasher could connect to the N900.

Solution
http://wiki.maemo.org/Updating_the_tablet_firmware

see the options in this section for a solution
Workaround for "no suitable device found" problem on Windows 7 x64