FIRST THINGS FIRST: Because adding a starter kit to your account requires logging in or signing up, it is currently only possible on a desktop device.
If you are on a on a desktop device, simply open the given starter kit url in your browser. The link should look something like this: deskish.com/kit/starterkit The starter kit will automatically be added once you've logged in or signed up.
To start using the content of the kit on your desk, open the tools (+) and click the library button (the one with the three dots "…"). The library is going to slide in from the left side. Now, select the newly appeared starter kit icon in the top part of the library and create away.