After open a app and tap, you'll see this screen.
Select Start button at this screen.

Then you see server selection screen.
Sonolus is a platform and you can't anything without add a server first.
(Like smartphones needs to download app)
Please select Add button under the custom server title.

FYI: The collection feature is for offline play and you need to download or import from server or scp file before use.

After you add, you'll back to server selection screen.
Here you'll see the server you added.