Okay so I know you need 30 lingots to get the bonus skills but awhile back when I tried to get them the duolingo store said: 0 slots available. Why is that? someone please tell me how to unlock the skills. Gracias!
You have to be a certain ways down the tree for them to become available. Here is a link to the Duolingo Wiki (unofficial) http://duolingo.wikia.com/wiki/Duolingo_Wiki#Getting_Help. You'll find a lot of Q&A here ^_^
I may be a bit late, yet my bonus skills' slots opened up, directly after I completed the Possessions' skill unit. :) Have you completed that skill yet?