Go to the "File" menu, then click "Export." A dialog box will pop up asking you where to save the export file.
Navigate to your EndNote data directory (typically, My Documents\endnote.Data). Make sure you are clicked INTO the .Data directory for your library. (This directory contains a PDF folder, but you should be sure to select the data directory rather than any sub-directory.)
This is important! Zotero will look for file attachments in a directory relative to the location of the exported XML file. If you save this file in the wrong spot, file attachments won't be included when you import into Zotero.
Under "Save as type:" (or "Save as file type:") choose "XML."
If you only want to export a subset of your library, check the "Export Selected References" box. Otherwise, make sure it is unchecked.
Import References to Zotero
Go to the "File" menu, then click "Import."
Navigate to the location where you exported your EndNote library (My Documents > endnote.Data).
Select the XML file. Click "Open."
Clean up imported notes:
If Zotero encounters any fields in the EndNote XML data that it does not support (e.g., custom fields, author address, author affiliation), it will add these data to a note attached to the imported item. These notes will be tagged with “_EndnoteXML import”. You should review each of these notes to determine if the data need to be retained (you can also move these data to the Zotero “Extra” field). Any unnecessary notes can be deleted. Additionally, you should check these notes to determine if any data could be migrated to proper Zotero fields (this is particularly important if you were using EndNote fields in non-standard ways).
You can quickly display all of the notes generated during import by clicking on the “_EndnoteXML import” tag in the tag selector in the lower-left corner of the Zotero window.
You can quickly delete all of these notes by selecting the tag in the tag selector, clicking in the items list and typing Cmd+A (Mac) or Ctrl+A (Windows/Linux) to select all matching items, and then right-clicking on a selected item and choosing “Move Item to Trash…”.