What version of MtoA?
In MtoA 2.1.x, just click an active AOV (AOVs tab of the Render Settings)
In pre-2.1 versions, do this:
Thank you very much.
And inside, I read on your website if I enable Merge AOV's, I can have all AOV's on a file with layers.
When I try, I have one file, but no layers.
Do you have <RenderPass> in your Image file prefix?
How are you viewing your AOV's? You may have a layered file but if you are viewing it in Photoshop or After Effects you will need to have a plugin-in like ProEXR or Exr-IO to view the layers.