The simplest way to add a marker is to search for a location with the Search Bar:
If you have many markers you may want to upload them by spreadsheet. This tutorial explains how.
A few notes:
Once you've added a marker you can adjust the latitude and longitude of it— just click to open the marker and toggle between address and latitude and longitude:
Atlist automatically gives markers a name.
Usually the name is the address but if it is a well known landmark (example: Eiffel Tower) Atlist will give it that name.
You can always change the name of the marker and it will not effect the markers location.