my paypal doesnt work, a few questions here

[lukelee]

I have read almost all the threats, but none of them work for me.
Here are my questions:
where is debug mode?
where do I turn it on or off?
when I want to do a ipn simulator, I have to change make
$url = "https://www.sandbox.paypal.com/cgi-bin/webscr";,  is this correct?
then when i click test through the PayPal IPN Simulator, I enter: http://www.mysite.com/administrator/components/com_virtuemart/notify.php  is this correct?
why do I get this output?:
IPN delivery failed. HTTP error code 500: Internal Server Error