You install Grunt on a per-project basis. Go to your project’s folder. It needs a file there named package.json at the root level. You can just create one and put it there.
The contents of that file should be this:
{
"name": "example-project",
"version": "0.1.0",
"devDependencies": {
"grunt": "~0.4.1"
}
}
Once that package.json file is in place, go to the terminal and navigate to your folder.