erp.inc - Undefined Function - theme_add_style

#It appears as though the drupal 5.x that I have no longer supports the function used in 4.x
#It was neccessary to change the line as below in order for me to install the module.
#
#Line 10 was changed from the deprecated function to the new replacement function
#
#theme_add_style($erp_base_path .'/erp.css');
drupal_add_css($erp_base_path .'/erp.css');