If you get an invalid username or password error, the cause is more than likely that your username is incorrect. When Minecraft first launched, they used your IGN as the username. Over time they started migrating accounts and switched to using your email address. So as a first step, make sure you try both your IGN and Email address as the user name. If that still fails, then you will need to use Minecraft to verify your credentials. If you are already logged in, you will not be able to see the user name. In the case you need to “Log Out” (see image below) and try logging in again. If this also fails then you must have forgotten your password. In this case you will need to use the “Forgot Password” feature and reset your password. Mojang will email you a new password and then you should be all set. Try it in Minecraft first and make sure everything works. Then you can use the same credentials in MineChat and be all set to chat away!