My boss bought this software. I installed it but now it is causing issues and I need to uninstall it. When I go to manage extensions it now just gives me a blank page. If he buys training will it help us do what we want? Do I install the plugin on the current site that I want to use the database from or the new slave site?

You can always buy our billable support to help you doing setup, install, uninstall, training or even develop specific things.

A "blank page" is generally due to a php fatal error.
Check if you have an "error_log" file with more detail on what may cause this potential "php fatal" error.

Theorically, JMS should only have to be installed on the master website and is not required in the slave site.
The only case where JMS can be installed in a slave site is in the case of a website creation from the front-end of a slave site. (Case of JMS full).