Try editing xorg.conf (sudo gedit /etc/X11/xorg.conf). Somewhere near the bottom is a section called "Screen" and should have a line of "Modes". Put your resolution on the end of that list and save, then restart X (press CTRL+ALT+BACKSPACE) then log back in.
I don't know why it's not detecting your monitor, probably cause it has an odd resolution. It works with my 1280x1024 CRT's and my 1600x1200 laptop just fine.