Might
this help with modmap?
$DISPLAY is presumably an environment variable just set it by doing:
$DISPLAY=<somethingorother>
You'll have to find out what from the manual or something. To check it's set do:
set | grep $
If you need to set $DISPLAY permanently put:
$DISPLAY=<somethingorother>
export $DISPLAY
I think, in your .bashrc or .bash_profile file, can't recall which one. Having said that the variable is only used inside X I think so it might be best put in ?.xinitrc or ?.xsession??
However it would be much nicer if these problems didn't arrise in the first place. So may be missing the real problem.
If it were me I'd first look in .xinitrc (is it?) or .xsession, in home directory, and see if it ends with 'exec <name of a window manager>'. Stick in something like 'exec fvwm2' (assuming fvwm2, a window manager, is on your path) to see what happens. Or remove everything except the final exec line.
An easy option might be to install Enlightenment, a window manager, because it has the alarming habit of overwriting .xinitrc with its own thing, or used to. Hope helps.