The toolbar in your screen shot looks identical to the one shown in the manual:
  https://www.nitrc.org/plugins/mwiki/index.php/mricron:MainPage
When you load overlays the drop-down bar that currently shows only "background" will show the additional layers.

While I am glad that MRIcron has proved popular, be aware that I am no longer actively working on this project, having shifted my efforts to MRIcroGL
 https://www.nitrc.org/plugins/mwiki/index.php/mricrogl:MainPage
and NiiVue
 https://github.com/niivue/niivue