Are you using the VoidLauncher? If so then you need to look at the bottom-left of the launcher window. There is a place to log in with your minecraft/Mojang account that you would normally use for minecraft.
Once you type these credentials in, you will be logged into minecraft with your account. There is an option to keep your account info from being removed that you can use, but if you don't want to have the launcher remember your info you can leave this unchecked.
Just remember that if you leave this option unchecked, you will need to retype your information every time you start the launcher.
Hope this helped,