As far as I’m aware, you shouldn’t need to do anything with certificate authorities for a development environment. CAs should only be needed for secure communication between nodes, which for most development work isn’t necessary – unless the development you’re doing involves federation.
I take it you’ve followed the installation instructions for Mac OS 10.13. If you haven’t followed that step-by-step, it’s likely you’ve done something wrong at some point.
Note that there is a not very obvious link after Bundler installation to Notes On Installing and Running Diaspora. You need to follow the instructions on that page to complete the installation, but to take note of the exceptions for Mac OS listed below that link on the Mac OS 10.13 installation page.
Bunder is a gem manager for Ruby. Once installed with
sudo gem install bundler, you install (and later update) the gems you need to run diaspora* with
bin/bundle install --full-index.
I’m not a dev, so I’d better leave it there for people more competent than I to help. But I hope it helps you find the missing steps from what you have so far done.
If you’ve noted any differences there are between the installation instructions for OS 10.13 and what you had to do to install on 10.14, you could usefully help to create an installation guide for 10.14.