They changed the delivery model for Karma recently

To install Karma itself, as you have already done:

npm install karma -g

But then you need to install drivers for testing frameworks. For example for QUnit

npm install karma-qunit --save-dev

Next you have to install launchers for the different browsers. For example Chrome and IE

npm install karma-ie-launcher --save-dev

npm install karma-chrome-launcher --save-dev

And now you should be good to go.

Now simply launch karma by using the start command with the config file as an input

karma start path/to/tests/karma.conf.js