Adding text to PDF using Adobe Reader in OS X
I guess we can assume that the document does not fillable fields (because if it had, the question would not have come up).

The "Add text at cursor" tool is a markup tool, used for correction text. It sets a marker in the text, and the text to add is contained in the popup.

The suitable Comment tool for this purpose ("filling" out a form) would be the Add Text Comment tool.