Go to the Informant settings, calendars triggers..., tap manage calendars.

Then tap the add button, and tap subscribed.

This will open a list of available calendars that you can choose from.  If you are not finding a calendar that you are looking for you can search the internet and locate a link for the calendar that you wish to use.   Copy the link into the subscription field in Informant and save.

The calendar should then be visible in Informant.

The exceptions to this would be any calendars that the author disabled the option for the calendar to be shared.