CMS Made Simple Forums

Is there a way to have a horizontal ruler <hr> button added
to the tinyMCE toolbar?

On their website it says that it is a plugin, but when I go to the plugins I don't see it to activate it.

You need to activate it in the plugins tab and you should have this, plus a lot more, in the profiles tab Advanced backend profile settings > Toolbar 2:
bold,italic,underline,strikethrough,advhr,separator,

I did this and see the HR button just fine...