yes, you need to publish the package to one of the package repos Parcel Service knows about. And since you can only publish released packages, Parcel Service will only deal with those and not with any new package still under development.
Packages are stored in Designer in two places:
a) as a set of objects in the Package Catalog of each project where you create and/or use them and
b) as feature and plugin jar files in <Designer install folder>/packages/eclipse.
When you import packages though Help - Check for package Updates... or by adding them in Parcel Service to the "Installed" virtual repo, they go as jars into <Designer install folder>/package/eclipse. After restarting Designer, it knows about them, and offers them for installation and updates inside projects opened with this Designer instance. Only when you first use a new package (or a new version of a package), it's jar will be extracted and the contents imported into the project.
So basically this is a two-step import process where the second step is transparent to the user. This used to be a manual step in earlier Designer versions (I think the 4.0.x versions) that has been automated only afterwards (beginning with 4.5, iirc).
It is also because of these two independent steps that you should regularly remove unused packages from the Package Catalog in all your projects.
Parcel Service does NOT access any of your projects (stored in the workspace folder and possibly shared via version control with other Designer instances) but only the packages your local Designer instance has installed in <Designer install folder>/package/eclipse