I am surprised you have not found mention of such software in previous threads because I have suggested one program several times. That is Empire Express by Haddon software. It is a basic track layout program that is easy to learn and use. Strictly 2D, but more than adequate for planning.
There is another called Railmodeller that is more elaborate.
I believe both have demos you can download.
Dante
-Dan
Builder of Bowser steam! Railimages Site
A quick check of Xtrkcad website indicates it is for Windows and Linux. Although Apple's OSX is based on Unix, as-is Linux, I have never heard or read that you can make a Linux program run on a Mac. Nor a Java program (no indication that Xtrkcad is based on Java, not to be confused with Javascript, a language used for websites and compatible, I believe, with all operating systems). But - you can always try! Nothing ventured, nothing gained!
However, common sense and computer experience should indicate that it is not wise to force such issues. Try Empire Express. If that is not fancy enough for you, try Railmodeller. Finally, if you want a Windows program, buy a newer Mac with the Intel chips and at least continue the benefit of using the best machines made.