You cannot mine Yellowcoin anymore, you can only buy it on Bittrex.com and poloniex. Must likely soon on cryptsy and mintpal
If you still want to own a few Yellowcoins then you can be part of their Miss/Mister contest. You just had to send a picture related to Yellowcoin and you will be able to earn some Yellowcoins as they will giveaway 10,000 YC each.
So have a look at
https://twitter.com/YellowCoinTeam