The Choose Package Dialog

In this dialog you select a package in the project, for a specific type of item (matrix, function...).
You can either:

select the package in the list and click on the Ok button.
double-click on the package

If the package is not in the list, you can add it clicking on the Enter Package button. It opens an input box where you can write the new package. Once entered, the package appears at the end of the list.

Cancel closes the dialog without a package being chosen.