Looks like you can download it here. An explanation why it wasn't included in the install.
https://pixologic.com/zbrush/downloadcenter/zplugins/ The plugins listed below are not included in the default initialization of ZBrush 2021. These plugins have had limited testing and could contain potential bugs. Please save your work before using any of them with ZBrush 2021! Since these additional plugins do not ship with ZBrush 2021, updates to these plugins may not be as frequent.
I loved the plugin when it was released. Since the 2021 release of ZBrush, I've noticed a large improvement in their current implementation of Dynamesh to the point that Dynamesh Master feels unnecessary. I may be wrong on that, it just seems the latest version of Dynamesh respects detail far more than its predecessor. It wouldn't surprise me if they worked in some of DMM's code base to the point that you really only need to test Dynamesh at a couple varied resolutions to achieve the same results as the old plugin.
Replies
https://pixologic.com/zbrush/downloadcenter/zplugins/
The plugins listed below are not included in the default initialization of ZBrush 2021. These plugins have had limited testing and could contain potential bugs. Please save your work before using any of them with ZBrush 2021! Since these additional plugins do not ship with ZBrush 2021, updates to these plugins may not be as frequent.