First I opened the template of ch2.nii.gz, then I wanted to overlay the aal.nii.gz on the ch2.nii.gz, but I can't find the aal.nii.gz at the add window in the MRIcron-Overlay option.
But I know the aal.nii.gz is in the MRIcron.app when I clicked 'show package contents' option. The nii file is in this file path '/Applications/MRIcron.app/Contents/Resources/templates/aal.nii.gz'.
How can I implement this operation?
The MRIcron is a MacOS version.

I would suggest using MRIcroGL
 1. File/OpenStandard/mni152
 2. File/AddAtlas/aal

You are correct, unlike other applications the MacOS applications are signed as a application package. You could always copy the contents of /MRIcron.app/Contents/Resources/templates/ to another folder. Note that you can also customize your contents of this folder to change the default settings. This approach works well on Windows and Linux, but may cause modern versions of MacOS to think the notarized application has been tampered with.

> I have done a alias of /Applications/MRIcron.app/Contents/Resources/templates. The alais can be loacated everywhere. That way I can choose the templates easly...

Isabelle - that is a clever solution. By the way, with MRIcroGL you can hold down the Control (or Command depending on OS) key when selecting an image to toggle whether it is loaded as a background or an overlay. I will consider adding this to the next release of MRIcron. I do encourage people to try out MRIcroGL, I am a full time scientist and have little time for these projects, so most of my efforts go to MRIcroGL which leverages modern hardware. I hope MRIcron is robust and mature, but it does feel pretty dated relative to FSLeyes and MRIcroGL.

Thank you very much

Well, I know this way, but I would like to know a way like I can open that aal.nii.gz in Windows system. Hah ^_^
 And thank you all the same.

On MacOS, the folder containing the image aal.nii.gz is hidden inside the MRIcron.app folder. For Windows and Linux users, it is in the \Resources\templates folder. You can simply open the aal image by using File/Open or Overlay/Add and selecting the image.