In the Designer click on AddView / CustomView.
You should see AnotherDatePicker.
Don't forget to copy also the DatePicker.bal file to the projects Files folder.
This is for the AnotherDatePicker Class.
A better solution would be to use the B4X library:
AnotherDatePicker.b4xlib
Copy the AnotherDatePicker.b4xlib file to AdditionalLibraries folder of B4A.
No need to care about the DatePicker.bal file, it is included in the AnotherDatePicker.b4xlib file.
Then you should see AnotherDatePicker in the Libraries Manager Tab:
If you still have the AnotherDatePicker Class module in your project, you must remove it!