Hi! Thanks for the feedback. Yeah you can import "samples" which are limited to 60 seconds right now, and drop-and-drop does not work yet, as you saw. There will be proper raw sample tracks for vocals in the future, but alas, not yet.
In the project explorer (right side panel) look at the top where it says something like "Project -> Song 1 -> Channel XYZ" and click on the project. This is like a folder structure you can navigate. At the root of the project is a list of samples, press the (+) Add button to load some. Then you can use those in a PCM sampler generator, but assigning them to the sample map . The documentation is extremely lacking regarding this and will improve in the future.
-Mat