Hi Paul.
Ok, your HD is too small for a Bootcamp / Windows partition so ignore that.
Your current macOS 10.11.6 El Capitan and MacBook are of a vintage that won’t have Gatekeeper so you can run un-notarised applications from any source, its only from mac0S 10.15.x Catalina that applications that have not been submitted to Apple by the developer and ‘Notarised’ are blocked from running. The developers have to pay a yearly fee to Apple for that service and many freeware developers are not prepared to pay. Apple introduced the concept of app ‘Notarisation’ and ‘Gatekeeper’ as a way to try and maintain security from malware and functional reliability of the macOS.
If you are uncomfortable running unfamiliar developer tools and software on a computer where security is important then to get started I would recommend using the ZWO supplied planetary imaging software for mac, you can find it at the bottom of the ASI camera software download page under the mac software tab, Capture Software, Planetary Imager:
https://astronomy-imaging-camera.com/software-drivers
The above is a safe video capture program that will run natively on the Mac with no extra libraries or support software required.
Once you have the planetary video file you can stack and combine the individual frames using the Mac freeware package Planetary Imager:
https://planetaryimager.gulinux.net
And for time exposure DSO imaging you can stack in ASTAP:
http://www.hnsky.org/astap.htm
If you want to delve into long exposure DSO imaging I would recommend a free trial of Stark Labs Nebulosity which will run natively on your Mac and has ASI camera support built in, if that works for you then a full license for Nebulosity is not too expensive. Nebulosity is one of the easiest capture and post processing suites to master.
For post processing there are several free and safe programs that are Mac compatible, including ImageJ which is science oriented, and Gimp which is a photo editing program although at the outset, Nebulosity alone can capture, calibrate and post process DSO images from ASI cameras to an acceptable standard all in a single app.
Many of the other free Astronomy Imaging apps for Mac such as SiriL require the use of Xcode/Homebrew to install.
It was not my intention to dissuade you from installing Xcode and Homebrew to allow Firecapture to be used on a Mac unnecessarily, only as a warning of the security risks that are taken where open source software such as Homebrew are introduced to the macOS environment since much of the built-in security of the macOS is bypassed. For an experienced developer or advanced user its not a problem to compare actual download checksums etc against the published repository values and to monitor which libraries and directories are being modified but it is not something I would want to do on my principle ‘safe’ computer without understanding what is being done.
Addendum Sun 26th Jan:
ZWO recently released a new suite of free capture software for macOS (and other OS’s), ASI Studio, that you may be interested in trying.
It only supports ZWO ASI cameras and filter wheels.
I downloaded this to my macPro this morning and gave it a try, it is not notarised for macOS Catalina but will run happily on your El Capitan system. My old series one ASI120MC does not support long exposure mode so I could not test all the functions in the suite but it looks as though it will support your camera. The suite contains a planetary capture app, long exposure DSO app and a live stacking EEVA app. It requires no special libraries or modification of the macOS and is simple to install. I scanned the mac download for malware in two different AV suites and it appears clean.
HTH
Link to the macOS version: https://astronomy-imaging-camera.com/software/ASIStudio_V1.0.dmg
William