To add an item, start off by clicking anywhere on the calendar to open this modal and make sure to switch to INTEGRATIONS.
By default, you would see only Points of Interests available.
However, TripCreator is able to offer you much more upon request, as an add-on to your account:
So, depending on the Integrations available to you:
to change the selected hotel rooms, click on the button of the selected option
all of the options will become active again and you will be able to choose a suitable one.
NOTE: changes can be made only before a booking was made.