Where is "texmf" on a Windows install?

'texmf' is not the exact directory name, but refer to the root paths of the TeX distribution. There are often more than one 'texmf' tree, and they are often not called 'texmf'. In MiKTeX, you can find all texmf tree in MiKTeX's option (Start menu -> MiKTeX -> Maintenance -> settings):

You could type at the command prompt:

kpsewhich -var-value TEXMFHOME

Alternatively:

kpsewhich -expand-var '$TEXMFHOME'

While on my Linux system, it returns /home/stefan/texmf, on Windows it may return something like C:/Users/Gablin/texmf depending on where your Windows user profile is.

The syntax of kpsewhich may vary depending on the system, for example regarding the quotes or dashes. Type kpsewhich --help.