This is preconfigured in a stock 3.2 installation. No need for any action on your part. You should be able to double-click an ADF and it will mount.
The specific mechanics of it are the file ENVARC:Sys/def_adf.info, which is a project-type icon with C:DAControl set as the default tool. That icon is used for all ADF files via the DefIcons system.
But if you have an ADF file with its own icon (e.g., mydisk.adf and mydisk.adf.info) you'll need to manually set the default tool on that to C:DAControl. The DefIcons icon is only used when an icon for a given file doesn't already exist.
EDIT: default tool, not tooltype