You have to add the package EnergyModelsGUI to your current project in order to run the examples.
You can run from the Julia REPL the following code:
# Starts the Julia REPL
using EnergyModelsGUI
# Get the path of the examples directory
exdir = joinpath(pkgdir(EnergyModelsGUI), "examples")
# Install required packages (use the project.toml file in the test folder of the repository)
using Pkg
Pkg.activate(exdir)
Pkg.instantiate()
# Include the code into the Julia REPL to run an example (*e.g.*, EMI_geography.jl):
include(joinpath(exdir, "EMI_geography.jl"))