I'm usually fine with the Harvard style, but since the text isn't really a published book, I have no idea how to reference it in the typical ancient style. For example, my entry for Plutarch is:
Plutarch. Plutarch’s Lives, trans. B. Perrin. London: Harvard University Press, 1914.
Can anyone recommend how to add a bibliography entry for the Twelve Tables?
Cite Plutarch. You don't have the 12 tables, you have Plutarch's account of them. Other ancient writers have different accounts, and modern scholars have reconstructions. You're using Plutarch here; that's your source. Ancient sources are usually cited by chapter and section (Plutarch, Romulus, 1.4 or something). The numbers are usually indicated in the margin of the page.
If you want a reference for a reconstruction of the tables, I think you can get it in either Crawford, Roman Statutes, or FIRA (fontes iuris Romani antiqui). These books, if I recall correctly, give each entry a number, and usually they are cited by title and number, not title and page.