Whats Going On Here?!

Im using Duolingo on my iPod Touch. It crashes and boots me out when Im in the Japanese course, and I try to go into the Store. Also, the website says where I can find Duolingo Plus in the Store. But Ive looked in the Store in each course Im taking and Ive never been able to find the link to buy it. I send bug report after bug report, and nothing happens.

LonelyHeartBreaking about Duolingo Learn Many Languages for Free, v5.1.7