His biography on the website of the World Economic Forum also refers to him as,
University Professor of Macroeconomics, Economics of Growth, Microeconomics and Mathematics for Economists.
However, I am unable to discover where he is a professor. The WEF biography only clearly identifies roles in industry, think tanks and as a government advisor. The papers he has published, that I have seen, either don't state his affiliation, or list it as Fundación Acordar, which is a think tank.
Searching for an answer to this has been hampered by the huge amount of noise caused by Milei's political prominence, but perhaps someone with better detective skills than me get to the bottom of it.