When endnotes display with roman numerals it is due to a setting in Word, not Zotero. To correct this issue do the following:
For Word in Windows:
- Under the References tab in the Footnotes section, click on the tiny arrow icon in the bottom right corner:

- Using the drop-down menu under Number format, switch to your preferred style of numeral; then click the Apply button (not the Insert button):

For Word on a Mac:
- Right-click into an endnote, then select Footnote... from the context menu:

- Using the drop-down menu under Number format, switch to your preferred style of numeral; then click the Apply button (not the Insert button):
