If you build on macOS, we offer organizations on our Free plan 30,000 credits per month to use on macOS builds, open source or not. This gives you access to the m4pro.medium resource class.
If you are building an open source project, these credits draw from a separate pool and differ from the 400,000 free credits provided to open source projects. The larger pool only applies to Linux jobs, such as Docker, ARM and Linux VM Machine jobs.
While not strictly needed to use the 30,000 macOS executor credits, to enable Open Source credits for your project, your project must be in a publicly accessible repository and have Open Source enabled in your projects settings.
Navigating to Project Dashboard > Settings > Advanced will show a toggle titled Free and Open Source, as shown below. This will enable the 400,000 Linux executor credit pool.