Two ways to get a message:
//At the top of the script add this
using GameSparks.Api.Messages;


void Start(){
//You need to subscribe to the ScriptMessage Listener because we are sending a custom message
    ScriptMessage.Listener += GetScriptMessages;

//You can do this for short stuff, but I prefer the one above as it gives you more control
        ScriptMessage.Listener = ((ScriptMessage message) => Debug.Log(message.BaseData.GetObject("data").GetString("messageNotify")));
}

void GetScriptMessages(){
    //Check to see if our data contains a "Type"
    Debug.Log(message.BaseData.GetObject("data").GetString("Type"));

    //If "Type" equals "ChatMessage", parse the information however you like!
    if (message.BaseData.GetObject("data").GetString("Type") == "ChatMessage")
        {
            Debug.Log("You've received a new message! It's from " + message.BaseData.GetObject("data").GetString("From") + " It was sent at " + message.BaseData.GetObject("data").GetNumber("When") + " It says: " + message.BaseData.GetObject("data").GetString("Message"));
        }
}