Only the recording parameters screen, change the button from stereo to mono. If you don't know how to do this, see my tutorial on getting started in MMM, where I show how to do this.
Assuming you have recorded the vocals using a microphone then unless you have used a stereo microphone the vocals are recorded in mono ie on one channel only.
You can fix this by right clicking the vocals, select Audio effects, Stereo processor and select the option to either Both channels = left or Both channels = right, as required, then use the balance control to position the vocals on the audio stage.