function makeDatum(string dataStr)  payable returns (bool) {
    debug("datum");
    data[msg.sender].payment += msg.value;
}

Here is my Datum struct:
    struct Datum {
        bool exists;
        bytes32 name;
        uint payment;
    }

data[msg.sender] += msg.value;

contract.methods.makeDatum("test").estimateGas({from: defaultAccount}, function(error, gas) {
    console.log("Gas estimate:" + gas);
    contract.methods.makeDatum("test").send({from: defaultAccount, gas:gas, value: 50000}, function(error, result) {
        console.log(result);
        console.log("error:" + JSON.stringify(error));
    });
});