OS X 10.11.5 (15F34)
Running: Java 8 Update 71

I downloaded ToughAsNails-1.0.1.jar and opened it. It fails to launch and after a few tries the system tells me

Nothing of interest is printed to console (I narrowed it searching for java).

I have a vanilla 1.9.4 profile which I opened at least once. It just doesn’t want to run properly. Is there any other way I can install the mod?

Huh? This is a Forge mod, not an installer.

Here's a quick summary of what you need to do:

1. Download the latest version of Forge for 1.9.4 from http://files.minecraftforge.net/maven/net/minecraftforge/forge/index_1.9.4.html and run Forge installer.
2. Create a brand new profile in the Minecraft launcher and change the version to release 1.9.4-forge, then run the game.
3. A 'mods' folder in your game directory should be created where you can drop in the ToughAsNails jar file, run the game again, then you're all set!

I feel so dumb. Thank you for correcting me and sorry for wasting your time!

No worries :)