You need to include the definition file for node.
TypeScript 2.0+
Install using npm:
npm install --save-dev @types/node
TypeScript < 2.0
If you use typings then you can run this command:
typings install dt~node --global --save
Or if you are using typings < 1.0 run:
typings install node --ambient --save
Or if all else fails, manually download the file here and include it in your project.