Immediately after reading this post I thought: "Damn, I use them for half a year and I'm really happy, I should support them" and went to website to upgrade to premium and... I couldn't find it! It was hidden as 4th option from top menu under small settings cogwheel on bottom.
Instead of all those useless (for me) statistics on front page you should put a small note like "Like us? Go premium!" to remind us that making something great needs support.