Hi, thanks for playing. So, about the issue you have I'm guessing you want to load a save slot and not import a save file that was previously exported. if you want to load a save slot you can follow these steps:
1. In the main menu click on Load
2. In the load game screen select a save slot
3. click on load
Here is a screenshot of the steps:
You probably have tried these steps and if they did not work, there might be a bug preventing you to load a save slot. And because you mentioned you are on Mac, please make sure you are running a chromium browser not Safari browser, to see if it fixes the issue.