Answer
The custom procedure shows up in the Method picker organized under the specific Component/Module your custom package was deployed against, then under the package's own PackageSpec entry — for example IFSApplications > [Your Custom Component] > PackageSpec > YourPackage_API — rather than in a flat alphabetical list. Navigate the picker's hierarchy by the component the package belongs to, not by searching for the procedure name directly, if it doesn't turn up in a plain search.