When you figure out how this works, it's great. Dev needs to put more explicit instructions right in the description. Most people won't have the patience to figure it out.

In the meantime: go to manage extensions, click the settings icon, click manage again, go to the preferences tab, and input a semi colon ; separated list of URLs you want as pinned tabs. Click save. Then when you open your next window click restore pinned tabs and they should all start loading.