Q. How can I transfer references from EndNote into Zotero (and vice versa)?

The trick is to ALWAYS export the libraries (from either the EndNote or Zotero citation manager) as a RIS (RefMan) file type. Simply “export libraries” from the original citation manager, then “import files” in your destination citation manager—the process is pretty much the same on both ends. Specifically:

Steps: EndNote to Zotero

1. Open the EndNote library you’d like to export to Zotero.
2. Navigate to File menu, then click “Export”.
3. The options menu will appear; make sure you know where you’re saving the exported library TO (e.g., create a special folder on your desktop called “Exports” so you don’t lose the information).
4. The file name will be the name of your library; the “save as type” will be .txt file; the output style should be “RefMan (RIS) Export”.
5. Save the file in your safe place (e.g., to a folder on your desktop called “Exports”).
6. Open your Zotero library.
7. Navigate to File menu, then click “Import”. For the option: "Where do you want to import from?" —Be sure that the "A file (BibTexX, RIS, Zotero RDF, etc.)" option is selected. 
8. Navigate to the “safe place” where you stored the exported EndNote library (e.g., to a folder on your desktop called “Exports”).
9. Select the exported RIS file, click “open,” and the files will populate the Zotero library you’ve selected, showing up in their own folder labeled “Import [Date, Time of import]”

Steps: Zotero to EndNote

1. Open the Zotero library you’d like to export to EndNote.
2. Navigate to File menu, then select “Export Library...”
3. A menu will pop up asking you to select a file format—again, RIS (RefMan) is the format you should select. Keep the default settings selected. I.e., [*] Export Notes, [*] Export Files, [*] Include Annotations; Character Encoding: -> Unicode (UTF-8).
4. Click OK.
5. Save the RIS file somewhere safe (e.g., to a folder on your desktop called “Exports”, so nothing gets lost).
6. Open the EndNote library in which you’d like to import the Zotero library.
7. Click on File, then “Import” and select “File”.
8. Use the menu to navigate to the place where you saved your Zotero exported library (e.g., to a folder on your desktop called “Exports”).
9. Select the file, then choose “Reference Manager (RIS)” as the Import Option.
10. Click “Import”.
11. The exported Zotero library should populate the EndNote library, showing up as “Imported References”.