In the 3.15 release of Altru, packages will not show up in your search results if they do not have an export definition attached to the package used in the Event Invitation process. Previous to 3.15, you could select a package to use in the Event Invitation mailing process without a letter or export definition.
The only required field for a package is name and export definition. As long as you have an export definition defined, you should be able to find this package when adding it to your event invitation. The export definition is required because it tells Altru what fields to output for you in your CSV file.