For those who come after, this answer is in another forum post, but it took me a while to find it. If you run npm install jsreport-wkhtmltopdf in your jsreport folder and get no node_modules directory and a short error that looks similar to this:

jsreport-server@ C:\Users\my-too-long\path-to-visual-studio-project
`-- jsreport-wkhtmltopdf@1.4.0

npm WARN optional SKIPPING OPTIONAL DEPENDENCY: fsevents@^1.0.0 (node_modules\chokidar\node_modules\fsevents):
npm WARN notsup SKIPPING OPTIONAL DEPENDENCY: Unsupported platform for fsevents@1.1.3: wanted {"os":"darwin","arch":"any"} (current: {"os":"win32","arch":"x64"})
npm WARN react-copy-to-clipboard@4.3.0 requires a peer of react@^0.14 || ^15 but none was installed.
npm WARN jsreport-server@ No description
npm WARN jsreport-server@ No repository field.
npm WARN jsreport-server@ No license field.

then my problem was that the path to the VS project was too long. Moving it up to something short like d:\my-vs-projects\ and running it gave the Done! wkhtmltopddf binary available at vendor\whktmltopdf.exe and a big long tree display of installed packages, and the node_modules directory with stuff in it.