Cannot Customize Using Yellow Pencil Plugin

I’m hoping someone can help me. I’m rather new to this –

I have my site setup – http://www.thewanderingpen.net and I am trying to customize the website using Yellow Pencil. I get this error msg –

Not Acceptable!

An appropriate representation of the requested resource could not be found on this server. This error was generated by Mod_Security.

I don’t know how to get into the code to edit there. I understand that if I edit in the CSS under Appearance that it will change once the template is updated. Do I need to purchase an upgrade of this template to do what I need to do? I’m not seeing an upgrade option.