While Zotero is free, you still have to register for an account. Use your @lion.lmu.edu or @lmu.edu email address for the account. Please confirm your email address as well.
Zotero registration page: https://www.zotero.org/user/register/
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Chrome:
Firefox:
Now that you have Zotero for Desktop and your Zotero browser plug-in, you need to sync your accounts. This will make sure everything shows up when you add citations. Think of it like Apple iCloud or Google Drive. You add a document or photo in one place and it shows up across all the platforms.