Hello and welcome to our forums,
In order to implement this scenario you can use a Property based folder. This folder will be resolved at install time to the values from the respective property. This property can be one of the Windows Installer Folders, the result of a Search operation or a custom property set in the Install Parameters page.
Here is the complete method:
-go to "Install Parameters" and create a new property named, for example "FOLDER". In the value filed enter the program directory path.
-go to "Files and Folders" page and create a new property based folder, based on "FOLDER" property
-add the dll in this folder
-as there are no other files in your package, you could go to dialogs page and delete "FolderDlg" which has become redundant.
As you are new, I would recommend taking a look on our
Creating a Simple package Tutorial
Best regards,
Sorin