Up at the top right next to your username, click your DEC balance

Next, enter the amount you want to transfer and click Steem Engine Wallet.

Then click transfer out.

I just realized this was the opposite of what you were asking.

In order to get your DEC into the game you just send it from your Steem Engine wallet to @steemmonsters and it will be put into your game account.

Now wait a few seconds and your DEC will show up in your Steem-Engine wallet. :)