A dialog box is displayed allowing you to ch1oose files you want to add to the package file. After selecting files you want to add, the files will be added to the bottom of the contents list.
Adding a file to a Package File can be slow or fast depending on the type of file that is open, the size of that file, the size of the file being added and the speed of your system.
If the contents list is being sorted, the files will not appear at the bottom of the list but will be sorted straight away. This is important if you are working with files that share common names.