Adding a wallet to NODE40 Balance simply means you are uploading the transactions from that wallet. It is important to note that transaction data does not expose any of your private keys and is safe to import to NODE40 Balance.
The images used in this article are from a Dash Core wallet running against testnet. The balance is not real Dash and has no value.
With your Dash Core wallet open click on the Transactions tab.
From the bottom right corner, click the Export button.
Give your export a filename (e.g. dash_wallet.csv) and save.
You now have a file ready for import into NODE40 Balance.