As a followup,
When I installed VS2010, it created a shortcut on my desktop labelled "Visual
Studio x64 Win64 ...". Clicking on that opened a command line window.
In that window, I could compile and run VC programs from the command prompt
using CL, LINK, etc.
In that window, if I typed:
SET
at the prompt, it showed the environment variables set, which included:
VCINSTALLDIR - where VC was installed
WindowsSdkDir - where the SDK was installed
This is all dmd needs in order to use VS2010.
However, I didn't want to have to click on that shortcut just to use -m64, so I
merely copied the values VCINSTALLDIR and WindowsSdkDir into my own personal
sc.ini, and then dmd -m64 worked fine.