Use the Extensions: Install from VSIX... command and to select the VSIX package you just downloaded
From Source
Clone this extension into the VSCode extensions folder (%USERPROFILE%\\.vscode\\extensions\\ on Windows, ~/.vscode/extensions/)
Just open a *.in file! if you don't see any syntax highlighting on what you think is a valid file, open the command palette (F1) and try "Change Language Mode".
Type in "spectra" to filter the languages and select spectra-in as your language of choice.
Feature requests
Just open an issue! make sure to provide details, screenshots, anything that'll get your idea across! If it sounds concrete and I have time, I'll implement it!
DISCLAIMER
I am not affiliated with Link Research Corporation or the SPECTRA 3d device simulator.
This extension was written for recreation over a couple of weeks, and to learn how language extensions work