Careful, there are two different import possibilies in ARaction, don’t confuse them:
A) Importing complete markers (including their content) from one project into another project.
B) Importing the content and settings of another marker into an existing marker.

A) Importing complete markers

1. In Edit mode, while no marker is selected, click on this icon:

If you need to add an optical marker to your project, click here:

… More info coming soon…

B) Importing content into existing markers

1. In Edit mode, select an existing marker by long-pressing it.
2. Click in this icon:
… More info coming soon…