First you need to place a "Secret" object which can be found under the "Logic" category.

My first thought was that you just had to step on the tile where you placed it, but nothing happens. What I did was to place a hidden pressure-plate on the same tile which is triggered by the party, so when you step on it, "Secret Area Found" will appear.
So place a
and put it at the same tile as the Secret tile, or put it right next to it so it will be easier to make a connection, then move it in place once you're done with it. 
Then click on the hidden pressure plate and add a connection to the Secret area by hitting the
button or just typing the name of the Secret area in the box. The "Action" should now say
.Also make sure to select the correct options you want for the pressure plate to be triggered.

These are the options I prefer, so it will only be triggered once, and so that the pressure plate doesn't make any sound, and it can only be triggered by your party stepping on it.
Now if you haven't already, just put the pressure plate at the same tile as the secret and you're done!

Hope this helped you!
Edit: Just after posting this I noticed that it was explained in the official Modding guide. Sorry lol. Oh well..

