[Plugin: WordPress Ultra Simple Paypal Shopping Cart] error when activating plugin

i’m running wordpress 2.7.1, and when i activate the plugin i get this message:

Fatal error: Call to undefined function plugin_dir_path() in C:\xampp\htdocs\wordpress\wp-content\plugins\wp-ultra-simple-paypal-shopping-cart\wp_ultra_simple_shopping_cart.php on line 32

can anyone help?

https://www.ads-software.com/extend/plugins/wp-ultra-simple-paypal-shopping-cart/