To enable a preinstalled application, you must know its package ID.
The list of applications installed in the system can be shown by the "AppList" app, which can be downloaded here.
Note: package ID in the picture is com.samsung.android.dialer.
After you get the package ID, open the Applications tab and click "Add".
Enter the package ID and name, set Version to 0. Select the "System" flag to indicate that this app is already installed in the operating system and doesn't require to be uploaded.
Click "Save", then choose in which configurations should this app be allowed.
This will do the task.