I noticed the link you posted was from an outdated version, see ⚡ Warpy - Whole Slide Image (WSI) Registration With ImageJ/Fiji, Elastix and QuPath for the current tutorial.

In the newer versions, they have simplified the installation process (likely that others were stuck at the same step you were), and have added an install_warpy.sh bash script to simplify the elastix installation as described in 1 - Installation | Warpy: Whole Slide Images Registration with QuPath and Fiji - YouTube.

Here’s the Github repo that has the aforementioned bash scripts: GitHub - BIOP/biop-bash-scripts: Bash scripts to simplify your life (let's hope so!)

To answer your original question as to where to place it, you need to unzip the Elastix application into the Applications folder on a Mac, assuming you were following Installation - Option 2 - Manual procedure, although I’d personally go with Installation - Option 1 - Automated procedure with bash scripts as described in ⚡ Warpy Extension

I’ll also ping @oburri if there’s anything he can add regarding installation on MacOS. As much as greatly value this tool, given that it’s one of the few open-source digital pathology alignment tools that goes beyond conventional affine/rigid-based transforms, I’ve never actually used it myself