You have to join the mingle challenge first. Then you posts and comments for coins. The mingle 18 deadline is today so too late. Join the new one tomorrow.
Nicole Comstock
Yes, if you say "I'm in" on the mingle post. Lots of people forget that you need at least 2 sentences for post and a complete sentence for comments.
Iris Oh
For mingle challenge, you can get upto 2500 coins.