source : https://www.zotero.org/support/word_processor_plugin_manual_installation

1.Open LibreOffice Extension Manager in Tools > Extension Manager.

2.Click Add in the Extension Manager dialog.

3.In your Zotero installation directory, navigate to zoteroOpenOfficeIntegration@zotero.org/install.

4.Choose the Zotero_OpenOffice_Integration.oxt file.

5.Restart LibreOffice and check if the Zotero toolbar appears under View > Toolbars