The listing, Emailed mCode for Mystery Moshling & trial moshi monster membership has ended.
This is a code for a game that we got in a Happy Meal (I assume). We don't play it, so I'm not going to be much help as to what exactly this is. I will email you the codes from both sides of the paper pictured.
One side has an mCode and says "Use this mCode to get the Mystery Moshling for your treehouse!" It also advertises McWorld at happymeal.com. Copyright both McDonald's and Mint Candy.
Other side is a code for a Free trial membership for moshi monsters at moshitrial.com. 3 day trial, and only one allowed per user account. Offer is provided by Mint Candy Ltd.
Expires 7/10/2014 (that's next year, not this!)