User-Font Resizer plugin for Atahualpa

Hi,

Can anyone recommend a font-resizer where visitors to the website can change the font size visible on their own computer screens?

I've tried "Font-Resizer" from cubetech (I prefer this font-size image) and I've also tried WP-chgFontSize from Ferran Rodenas.

Both are easily installed -- but they just don't seem to work on my website.

Both also mention that a php line of code can be inserted somewhere in the Theme. But where? And how?

As I am not a programmer and new to website building, is there an alternative that works with the Atahualpa theme?

Thanks

Janine