Meteor needs the MONGO_URL to be defined before startup functions are called via Meteor.startup. You should define your MONGO_URL environment variable before starting Meteor. If you’re using Linux/OSX for example, you can just specify it before the meteor command, on your command line. So on the same line in your console, just type in the full line I listed above (substituting in your proper MONGO_URL).
Since you’re using Windows, run the set MONGO_URL="..." part first to set the environment variable, then after that’s done call meteor on a separate line in your console.
Just to add to this, you may need to remove the quotes around the URL on the set command. Windows can be somewhat literal and I recall getting involved in a post where the quotes were being passed as part of the data.