Let's start at the beginning:
Steem pays you interest on your SteemDollars!
The interest rate is set by the witnesses and is currently 10% (to encourage holding/buying).
When is interest payed?
According to the code
Every time the sbd_balance is updated the sbd_seconds is also updated. If at least STEEMIT_MIN_COMPOUNDING_INTERVAL_SECONDS has past since sbd_last_interest_payment then interest is added to sbd_balance.
Where STEEMIT_MIN_COMPOUNDING_INTERVAL_SECONDS is set to 30 days. These 30 days have now passed.
That means:
If you change your SteemDollar balance today, you will receive your interest for the last 30 days!
I simply moved 1SBD to one of my other accounts and my interest was added to my account:
If you don't have another account, you are free to send 0.001 SteemDollar to me (or anyone else) :D
Update 1
@ash made me aware of the fact that initial interst payments have happend on different times. That means that you might not yet be able to retrieve your own interest just yet.
However, the first 30 days have probably past for many Steemians already so it's worth a try. If it doesn't work for you, just wait another day or so and try again.
Update 2
Since this post has received quite some response, I added a corresponding call to piston. It looks like this:
$ piston interest xeroc
+---------+-----------------------+---------------------+---------------+-----------+
| Account | Last Interest Payment | Next Payment | Interest rate | Interest |
+---------+-----------------------+---------------------+---------------+-----------+
| xeroc | 2016-08-03 08:04:00 | 2016-09-02 08:04:00 | 10.0% | 1.046 SBD |
+---------+-----------------------+---------------------+---------------+-----------+
piston.web now also shows some data about the interest payments:
