Hi,
I suppose the plugin you have installed also works for the HikaShop login form.
However, the text of the form still says "username", but if you enter the email address in it, it will probably work.
In that case, you can just use a translation override to change the text:
www.hikashop.com/download/languages.html#modify
And on the registration form, you can also remove the "username" field by changing the "registration" setting to "simplified registration with password" in the HikaShop configuration. In that case, HikaShop will automatically use the email address of the user as the username and thus even without any plugin, customers will be able to login with their email address as username.