try this:
you were clearing the current buffer after drawing to it and you weren't Flipping it to the front.
Graphics 800,600,0 'set gfx mode Local x=0 SetColor 255,255,255 Repeat Cls 'clear the buffer x:+1 DrawText +x,100,100 Flip 'flip the backbuffer to the front to display what we've drawn Until x=400
you were clearing the current buffer after drawing to it and you weren't Flipping it to the front.