While all common browsers have an option to change font size, not everyone knows that. But buttons make it easy for those that are not computer savvy. So if your website audience is expected to be older and/or less computer literate, why not make it easy for them to increase the font size?
Paste this into the head section of the page. Do you see near the top of the script, where the container to modify has been selected? $('#ContentColumn'). That means everything within #ContentColumn will change.
The buttons can be pasted where you think best. I suggest the include above your main headline. If you want to style the div for these, then create #switcher in your stylesheet, and style the div as you please.