Every action in the Nerve app requires Ethereum to succeed.
After logging into your wallet, navigate to the wallet history display by clicking on the two arrows in the Navigation Bar at the bottom. Click on the "Receive" button to get your Ethereum wallet address.
To get Ropsten testnet ehereum: Go to https://faucet.ropsten.be/ or to a Ropsten faucet of your choice and copy your wallet address into the textfield. Click the button to get Ropsten eth until the succeeded message appears.