How to access Userdata within a plugin

Hello,

I am trying to write a plugin. And i have added an extra field in a registration form using the plugin successfully. I would like to check whether the value in the extra field matches any username already registered.

When i try to validate using the predefined usermeta functions. I get the below errors:

require( ‘C:\wamp\www\cpa\wp-load.php’ )
require_once( ‘C:\wamp\www\cpa\wp-config.php’ )
require_once( ‘C:\wamp\www\cpa\wp-settings.php’ )

I would like to know how to access all the predefined functions of wordpress inside a plugin like we do in theme files.

Thanks.