I have emerged xdm but rc-update add xdm default gives '/etc/init.d/xdm' not found.
How can I get the script into /etc/init.d/xdm?

Something may be amiss here...did you verify that the xdm emerge went ok? Does ls /etc/init.d/ show any dm?

I don't remember seeing any errors on the xdm emerge but there is no xdm in /etc/init.d.
Should I try it again do you think?

That script is not part of xdm, as odd as it might sound.

xdm has actually two meanings:

• xdm, the init script/service. It's a service that enables graphics logins by calling a display manager.
• xdm, the display manager. It's one of the many display managers that exist, along with kdm, entrance, gdm, and a few others.

So, the xdm service can call the xdm display manager, but also any other display manager.

The x11-apps/xdm package contains the xdm display manager. The xdm service is contained in x11-apps/xinit. The mystery now is "why the hell isn't that package already installed?". That package should have been installed as a dependency when you installed Xorg. It doesn't only hold the xdm init script, but also some very important files like startx or xinit...