In order to trade ETH on Radar Relay, you will first have to wrap it.
Why do I need to wrap my ETH to trade?
For the time being, relayers can only trade ERC-20 tokens. The ERC-20 standard was created after Ethereum was made so ETH doesn't conform to it. In order to make it meet the standard is must be "wrapped".
For more information on WETH and why it's needed visit WETH.io
How to wrap ETH
Open the wallet drawer and make sure you're on the "Your Tokens" tab.
In the field under ETH enter an amount of ETH you wish to wrap
Make sure to leave enough ETH unwrapped in order to pay for transaction fees. They can't be paid in WETH.
Press the WRAP button, choose your gas price and then confirm the transaction on your wallet.