Before you can import a wall, you will want to open an existing RetainPro Project File or create a new one.
Then you will need to get to the opening screen that lists the wall calculations that are currently in the selected Project File. If you have any walls open for editing, simply "Save" or "Cancel" that wall and you will be brought to the correct screen.
At the top of that screen, you will see the "[Import] button. Click that button and you will be presented with a screen in which you can navigate to the Project File that contains the desired wall calc. Select the appropriate file then click "Open" to get a list of walls.
Select the wall you wish to import then click the "Import" button. That wall is copied from the source Project File into your current Project File.
You are now able to edit that wall in the current Project File.