I am using Unity (2018.1.3) on Windows PC and I would like to build for Mac, can you tell me how to do that?

I tried to make an application for Mac.
So, I attempted to make a setting from “Target Platform” in “Build Settings”, but Mac did not exist as a choice.
I thought that I could get something about Mac builds from “UnityDownloadAssistant - 2018.1.3f1” and I ran it, but there was not anything in Mac OS there too.
The OS I use is Windows 8.1, the version of Unity is 2018.1.3f 1.
I am sorry for my poor English because I am Japanese.
I would be pleased if you could tell me.
Thank you.

Did you select Mac as an option when you installed? Otherwise it’s not available until you do. Rerun the Download assistant and select the OSX and Linux standalone build options to download and install.