Make sure you've selected "This home is on other platforms". Next, follow these 2 steps:
1. Copy Plum Guide’s iCal from the button and import it onto your other platforms.
2. Export the iCals from the other platforms and paste it on the “Calendar URL” field. The iCal should always have “.ics” in it. Should the iCal not be successful in syncing, an error message will appear on the next step.
For more details, please watch the video below (mostly from 0:48-2:12).