The short answer is: nowhere. If you previously had an official Debian Openbox .deb installed and saw the Debian Menu in the root menu, the files are all still present on your system. Openbox.org .debs and/or source installs don't include them in your root menu -- that's done by Debian developer magic. You can do it with a few simple steps in these instructions.
How do I disable the popup when switching desktops?
Please see the upgrading to 3.4 guide.
How do I run Openbox across multiple X screens?
In order to have Openbox manage multiple X screens (this is not the same as multi-monitor TwinView or Xinerama), you need to run an instance of Openbox directly on each screen. We've put work into making Openbox work well with other instances of itself, for this type of configuration.
In order to run Openbox on two screens, use commands such as these:
# run openbox on the second screen (they start from 0)
DISPLAY=:0.1 openbox &
# by default openbox will run on the first screen (screen number 0)
exec openbox-session
How do I make Tweetdeck/Other Adobe Air apps work under Openbox?
Just put:
export GNOME_DESKTOP_SESSION_ID="openbox"
in your autostart.sh or .xinitrc (the "openbox" part can be any word). You can then run tweetdeck/twhirl etc. without any problems, although you might needs some gnome keyring elements installed.