As the link posted by @j-sowwy is showing 404 error (page not available), I couldn't see the instructions there but probably that page was to doenload xlwings.xlam file for installation of the add-in. Let me clearly write how to install the add-in for beginners:

1. Package xlwings (Open Source) requires an installation of Excel and therefore only works on Windows and macOS. It comes pre-installed with

• Anaconda (Windows and macOS)

• WinPython (Windows only) Make sure not to take the dot version as this only contains Python.

Else, you can install via pip, conda or cona forge as mentioned on the official doc.

2. The xlwings Excel add-in requires the xlwings Python package and Excel to be installed. The ribbon of the add-in is compatible with Excel >= 2007 on Windows and >= 2016 on macOS.

To install the xlwings add-in, the simplest way is to run the following code on Anaconda prompt or command prompt (as per your settings) (Windows) or terminal (macOS):

xlwings addin install

You will the a new xlwings tab in the ribbon (looks slightly different for windows and mac users). Here's screenshot of the ribbon on windows machine.

The macOS doesn't yet support UDFs and do not have a section for conda as it allows you to set conda env in Interpreter (python section). For more details of configuration, check official doc here

How it works?: The actual installation of the add-in is done by copying xlwings.xlam from the directory of the Python package into Excel’s XLSTART folder. Excel opens all files that are in this folder every time you start Excel.
Means, if you have xlwings.xlam file, you can also install the add-in directly from Excel the way you do for other add-ins: File → Options → Add-ins → Manage: Excel Add-ins → Go
Earlier, I could see xlam files available on package GitHub but probably they are removed. I can't see those.

NOTE/Warning: The add-in needs to be the same version as the xlwings package. Make sure to run xlwings add install again, if you upgrade the xlwings package.