I'm not a mac guy, but afaik this tutorial is pretty well written: https://bitzuma.com/posts/how-to-verify-an-electrum-download-on-mac/
You may check if you didn't forget anything (like importing ThomasV key).

The Electrum site reports his key ID as 0x2bd5824b7f9470e6. Use this value to look up Voegtlin’s public key. Click the GPG Keychain “Lookup Key” button and enter the developer key ID. The[n] click Search.