Dunno about the puzzle, but you could try changing the font (look above the italisice/underline buttons) to Courier New, which I believe is a Fixed Width Font. It's a bit thin, though, so try bolding it.
Like this:
XIXIXIXIXIXIXIXIX
~----------------
XIXIXIXIXIXIXIXIX
~----------------
XIXIXIXIXIXIXIXIX
~----------------
XIXIXIXIXIXIXIXIX
~----------------
XIXIXIXIXIXIXIXIX