Recently I am being receiving many queries regarding using Keymagic to write Malayalam on Mac OS systems. This is the post I write as help for those people, also to help myself to avoid writing separate instructions in each reply.
I do not have any experience or access to Mac OS based system now. So, my writing here may not have enough details. However, I assume it will be useful. Please let me know things I need to improve in this post as well.
First you need to download KeyMagic 1.5.1 - OSX Installer from this download page. Install that package in your Mac OS system. This Keymagic package comes with just Burmese language support, I guess. Now you need to add Malayalam writing methods there.
Next, download Malayalam-InScript.km2 and Malayalam-Mozhi.km2 files from this page. Then copy that files under Keymagic installation folder along with the Burmese language .km2 files.
Here is the comment from Suresh Manimala regarding finding location where to put the .km2 files.
Macintosh HD/Library/Input Methods/ keymagic. Now what you see is the key magic icon. Right click on this icon and select "Show Package Contents". A folder named "Contents" appear. Open it and Click on the subfolder "Resousres". Then paste all the .km2 files into this folder "Resources". Restart. Things will be fine.
As said already, restart your system and start using Keymagic to write Malayalam. Happy writing in Malayalam!