I've installed meteor on my machine:

curl https://install.meteor.com/ | sh

My understanding is that it meteor runs off of node.js and automatically installs it. And node automatically installs npm.

I'm working through the Discover Meteor tutorial and it has me run:

npm install -g mup

But I get the following output:

-bash: npm: command not found

Do I need to run it from a different directory? Or download / install something extra onto my machine. Add it to my PATH?

When you install Meteor, it does not automatically install node && npm. When installing Meteor, it will download something called dev_bundle which has NodeJS and all the NPM modules needed by Meteor. All these modules are pre-compiled for your platform but are not "installed" per-se.

Check out this post if you want to use the node distribubtion included with Meteor: https://meteorhacks.com/how-meteor-uses-node. I suggest you just install Node on your own, though.