Error downloading Lumen via Composer
My guess would be that you have already added a package globally that usedsymfony/process ~3.0.0 but Lumen installer requires symfony/process ~2.3 so composer cannot install both.
You can either try to resolve this yourself by removing that package and installing Lumen installer globally, add the installer to your working directory (i.e. leave out the global keyword and you should be able to use it in that directory) or Lumen provides a way to install itself directly, by simply running:
composer create-project laravel/lumen YourProjectName --prefer-dist
in your installation directory.
Checking out the latest commits for lumen installer https://github.com/laravel/lumen-installer. It appears there was a recent fix to the installers dependencies (i.e. symfony/process ~2.3|~3.0).
If you're keen to use the installer sooner rather than later get the dev-master versioncomposer global require "laravel/lumen-installer=dev-master" until there is an official release. Otherwise you can wait until the fix is properly released.