You have to add the package EnergyModelsGeography to your current project in order to run the example.
It is not necessary to add the other used packages, as the example is instantiating itself.
How to add packages is explained in the Quick start of the documentation.
You can run from the Julia REPL the following code:
# Import EnergyModelsGeography
using EnergyModelsGeography
# Get the path of the examples directory
exdir = joinpath(pkgdir(EnergyModelsGeography), "examples")
# Include the code into the Julia REPL to run the network example
include(joinpath(exdir, "network.jl"))