The deployed contract has a function as below:

function checkBalance() onlyTrainee constant returns (uint256) {
    return traineeBalances[msg.sender];
}

After interacted with the contract on MyEtherWallet, I call the checkBalance() but there is no input filed appeared that let me to fill with who I am (e.g. {from: '0x....'}).

Is there any way to ask MyEtherWallet to know who the caller (msg.sender) is?