The N900 image currently does not come with a X window manager. It is however sometimes needed in other to develop in a proper manner. In the MeeGo repositories, there is a package for the TWM window manager which you can install. You will need to have USB networking up and running in order to do this in your X-Terminal:
yum install xorg-x11-twm
In x-terminal you can then run 'twm &'. If you would like to run twm at startup, you can edit /etc/sysconfig/uxlaunch to point to twm binary (use 'which twm' to find the path)