There are two reasons you're not appearing on the leaderboard:

1. You've selected not to appear on the leaderboard in your profile settings.
2. You have trades with unrealistic leverage.

1: Profile setting:

Go to your profile by clicking the blue dropdown button in the top right hand corner and then on "User Profile".
On your profile page, click on "Update Profile".

Scroll all the way down and make sure the "Add me to the Leaderboard" checkbox is ticked.
Click on the "Save" button to save your updated profile.

2. Unrealistic trades

The whole point of demo trading is to learn how to trade.
In essence, you should be trading the way you would if you were trading an account with real money.

The first and most important aspect of successful trading is risk management.
This is done by placing your stop loss at a price level that would result in an acceptable loss, as well as using appropriate position sizing or leverage.

If you have any trades with an unrealistic position size or irresponsible leverage, you will not appear on the leaderboard.
In your trades table, under the Den, you'll see the trades with unrealistic position sizing highlighted in red as shown below.