This plugin requires a manual template modification in order to work, unfortunately. So we have a few options.
1) Don't install it. Let users increase the text size with their browser controls.
2) Install it, activate it networkwide, and put the template modification in the sitewide footer. That way it will be available on every site.
3) I can extend the plugin to include a widget that will allow blog owners to add the controls in a widget, rather than requiring the modification of a template.
I think (2) is a bad idea. Don't want to activate something on every blog (adding to load times, etc) unless there is a compelling reason to do so. Plus, hiding the controls in the footer would virtually guarantee that they are rarely found or used.
(3) is not a terrible solution, but it might be confusing for blog readers to see the controls on some blogs but not on others.
Essentially, I think that (1) is the best solution. Text size is and should remain a client issue. Reproducing a browser function on the page is a little like embedding Back and Forward buttons on every page: technically possible, and perhaps convenient for some, but out of place and redundant for most.