You can operate PDF2XL in one of two modes: Layout Editing Mode, and Text Selection Mode.
You can switch between the modes via the Marking Mode panel on the Home or Content toolbar.
Layout Editing Mode
In this mode you can add new elements to the layout, or select and edit existing ones. If you have selected "By Content" on the Marking Mode panel, then you are in Layout Editing Mode.
To create a new element, simply click anywhere on the document (not on an existing element) and drag the mouse over the area you want to mark. By default, PDF2XL will attempt to automatically guess whether the marked area should be a table or a field, but you can set the marking mode to "Field" or "Table" instead of "By Content".
To select an existing element, simply click on it. You can select both tables and fields regardless of the current marking mode. Once selected, you can move or resize elements just like you would any application window on Windows.
Text Selection Mode
In this mode you can select text from the document and copy it, but you cannot interact with existing elements or create new ones.