What do you want to do? Texture paint on the eyes?
Select the Manuel Lab model in Object mode. Go to Edit mode. Deselect everything if it was selected (A). Mouse over the body, press “L” to select body only and then press “H” to hide the body. Press “5” numpad key to change to orthographic view. Mouse over the eye, press “L” to select and press “H” to hide external part. Repeat with the other eye. Mouse over again on the eyes and select them with “L”. With your eyes selected, go to Texture Paint mode. From below you press the button with a small cube with red dotted front next to render layers arrangement (Face selection masking for painting). You configure symmetry in X axis, and you verify that you are painting over the correct slot in Slots tab.
According to what I know, Manuel has collaborated for a long time with MakeHuman project.
Oh, I read “The skin shader is a part of eyes”
If that’s not what you wanted to say, I do not understand what you meant in that sentence.