These steps work for both Zotero and Mendeley citation management programs. Screenshots show a Zotero library.
1. Open your other citation manager library, click on the File > Export Library. Choose Export format as RIS, then click "OK" to save the files.
2. In EndNote, click File > Import > File.
3. Click the "Choose..." button and select the file you exported. Set Import Option to Reference Manager (RIS) and click Import.