Hi DonR1 and welcome to Tiki Community!
(and same to you, QBass, if nobody welcomed you when you first arrived )
Tiki has a different Model than many other web appls. out there: all-in-one. So that all plugins (whre there is no issue with copyright licenses on plugins adapted from other systems) are built and distributed inside Tiki already.
So for Plugin Html, you need to enable it as an admin through "Admin home > Editing and plugins", and in the second tab, you will see the plugins enabled and disabled, etc.
In addition, there are a few other plugins (because of license issues) kept in:
(where you can download them, if you need any extra one)
Cheers, and thanks for helping other users in tiki forums! (otherwise, it's not scalable nor sustainable)