The Plan 9 distribution floppy is configured to bring the system up in CGA mode and without the mouse enabled. To enable the mouse you will have to run the command aux/mouse which takes one argument specifying the port the mouse is attached to. The choices are 0, 1, or ps2. For example aux/mouse 0 will set up the mouse on port 0. The aux/vga command will switch to one of the vga modes. It's argument is the name of a file specifying the VGA parameters. Look at the files in the directories under /lib/vga. Unfortunately there is no one configuration that works on all machines. However aux/vga generic/640x480x1 works on most vga card and monitor combinations. You may want to edit /rc/bin/termrc to have aux/mouse and aux/vga run whenever you start Plan 9.