Arrivals and Checked-in Reports are found inside the Reports function inside the Statistics Tab. You can click Here for the tutorial about the Statistics tab.
It will show you all the arrivals and check-ins per day for a specific day.
You can customise your report by choosing ON what date or BETWEEN what dates you want to view. Also you can filter the guests by status ("waiting for guest", "checked in", etc). Please remember to hit "Search" after making some changes to the report you wish to view.
It will show the guests' room number, name, source, status, booking date, check in and check out date, room type and their outstanding balance.
You can also change the status of the guest, export a CVS (spreadsheet) version of the file, and print a copy of the report inside this function.