Thanks for your question. I found this plugin in the WordPress Repository, and I assume it is the one you referred to:
User Role Editor
By Vladimir Garagulya
On my system, without that plugin or any modifications, the Editor role is able to add taxonomy terms without any problems. If that’s not working on your system, let me know and give me more details so I can investigate further.
To answer your specific question, the capability you need to assign to your Editors is edit_terms. I don’t know why this capability does not appear in the Codex but it does come up in support topics like this one:
Allow author to edit custom taxonomies
There you can see that there are several capabilities relating to taxonomy terms. None of them show up in the User Role Editor list but you can add them manually.
I hope that gives you the results you seek. I am marking this topic resolved, but please update it if you have any problems or further questions regarding the above suggestions. Thanks for your interest in MLA.