The Offerwall is available through the Store under the Buy Hashcoins section when you tap the 'Free' button:
These offers are provided by our partner, IronSource.
There may be a featured offer at the top. Scroll to the bottom to load more.
Once you complete an offer, the Hashcoins should be credited to your account typically within a half hour.
If it's been longer than an hour and you still haven't received your Hashcoins, check out I completed an offer and didn't get my Hashcoins?!