Buying directly from the BigWheel screen is not correctly implemented.
Buy your W-Coints from the "Item Mall" tab. You will find it under "Promotion"->"Recommended" and then on page 3.
You can get 1 piece for 150 IM Points, or 5 pcs. for 650 IM Points.
After you purchased that you can simply press the "Start" button on the "BigWheel" tab.
For every time you press "Start" one W-Coin is consumed.
Good Luck.
Edit: Since you are new, you can't post more than 3 posts in the first 24 hours. If you have any further problems, feel free to contact me by PM.
Edited by VModPopcorn, 16 October 2014 - 02:03 PM.