OK, it's not as hard as you think. To buy tickets through Indievox, you must first register and create an Indievox account. Then you can click on a film to place an order. The website will give you a code which you can take to any 7-Eleven, where you can pay cash and immediately get your tickets. Here are step-by-step instructions:
1) Go to Indievox main page, scroll to the bottom, change the language to English
2) Order your tickets online here.
3) Find the film you want to see (make sure it's the correct date)
4) Click on the red button that says BUY TICKET. If you don't have an account, create an account.
5) Pick the quantity of tickets, then click SUBMIT
6) You'll get a screen that looks like this:
7) Write down your account number and ticket retrieval code
8) Go to 7-Eleven. At the ibon machine, clicking the green button will lead you to a screen with the 2nd button, which will lead you to a screen with the 3rd button. If that's too confusing, just ask the clerk. They are used to helping confused foreigners.
9) Pay the clerk and get your ticket. Come to Urban Nomad and have fun!
FINAL NOTE: Your order will be cancelled if you don't buy your ticket within 24 hours. There is a NT$30 charge (from Indievox) for each purchase.