How do I get Microsoft fonts (specifically Khmer UI, Khmer Sangam MN) to load and be useable in InDesign and Illustrator? I see and use them in Microsoft but they are not working well in InDesign. Some characters are missing. I am on a Mac not a PC, btw. Thank you!
Some characters are missing? I don't think that this sounds like a case where the font is not showing up in InDesign, right? I suspect that you're working in Cambodian and haven't turned on the World-Ready Composer. If the font shows up in the list, it's installed. If it's not rendering your Khmer text as you'd expect, it's probably because you're not using the correct Composer:
This can be set in a bunch of places (in Paragraph Styles, in Type -> Apply World-Ready Composer, the Justification dialog I've shown here, etc.). That's where I'd start. If this isn't the issue, can you please provide some more detail?
Also, please note that Khmer Sangam MN is not a Microsoft font, but part of OS X's infrastructure for displaying Khmer. I think it's an AAT font (Advanced Apple Typography) and I have no idea how well it works in InDesign, or if you'll be able to get it to work at all.
Missing font has nothing to do with the composer.
Just to add to Joel's comments, are the fonts showing up in InDesign's font menu? If so, it's likely the composer. Also, I recommend taking a look at Google's Noto fonts. They are free and have an enormous range of language/glyph coverage. We use them often. https://fonts.google.com/noto
Microsoft saves its fonts in their application.
On the Mac you do following, to get MS fonts: