It is not instantaneous. The plugin waits for paypal to send back a payment notification. This only happens after the paiement is done on paypal. It can take up to one minute.
Is the file components/com_hikashop/upload/safe/logs/report.log writable ? Normally, you should get reports in that file when paypal sends a notification. If you don't there are only two possiblities:
1. the file is not writable
2. paypal didn't trigger the plugin
Also, when paypal triggers the plugin, in order to verify the request, the plugin opens a connection to paypal. That process can be blocked on some hostings. In such cases, however, you should get a notification email to the email specified in the option "payment notification email" under the tab mail of the configuration.
In order to rule out problem 1, pleas try to open with your browser the URL:
http://your_website.com/index.php?option=com_hikashop&ctrl=checkout&task=notify¬if_payment=paypal&tmpl=component&invoice=ORDER_ID
after replacing the website name and the ORDER_ID by the id of one of your orders. You should get a notification email and the log file should be filled with information. If you get errors regarding the connection to the paypal server, you should ask your hosting company to unblock outside connections.
If you don't get anything in the log, then you have an access rights problem on it.