The Mac provides a MPK starting with xpub
your Mac wallet is creating a
MainNet wallet for you if the master public key starts with "xpub". the master public key prefix for TestNet is "tpub".
in your Mac you have to run the Electrum instance with
--testnet command appended to it. i am not familiar with Mac, if there is a console or something run
that should do the trick.
edit: this might help:
https://bitzuma.com/posts/a-beginners-guide-to-the-electrum-bitcoin-wallet/#testnet-on-mac