I got your email and have responded to it. The reason you are getting an error is because you are trying to install the Nice PayPal Button for Wordpress into you Joomla site. Wordpress plugins are not compatible with Joomla. Wordpress plugins are for use with the Wordpress system. For more information on Wordpress see the link below.http://wordpress.org/
Anyway, I sent you the correct plugin for Joomla. It is attached to the email I sent. Please watch for it in your inbox.
~ All the best