No PayPal IPN Sandbox at Checkout confirmation stage ?

I signed up with PayPal for the developer section and have set my store up to use a fictitious Sandbox PayPal account. I've created two sandbox accounts to make some test payments.

My problem is when I go through the checkout process I can't use my activated Sandbox account since the store automatically forwards to regular PayPal and as such I can't use the test accounts I created.

What critical step have I missed?

set this
Mode for PayPal web services

Default:
https://www.paypal.com/cgi-bin/webscr
or
https://www.paypal.com/us/cgi-bin/webscr
or for the UK,
https://www.paypal.com/uk/cgi-bin/webscr
Choose the URL for PayPal live processing

to sandbox.paypal.com/cgi-bin/webscr

Thanks for the quick response Merlinpa1969

I'm from the UK, I don't know if that affects the information I have to fill out but I tried:

sandbox.paypal.com/cgi-bin/webscr

and

sandbox.paypal.com/uk/cgi-bin/webscr

After I click checkout I get the following message in both of the above cases

"The server at sandbox.paypal.com is taking too long to respond"

I'm usingZen Cart 1.3.6

FIXED

The solution was a little misleading, it lacked the www. prefix and I assumed the URL you gave was correct

For anyone else having the same problem use the following setting up Modules > Payment > Paypal

www.sandbox.paypal.com/cgi-bin/webscr for the US

and

www.sandbox.paypal.com/uk/cgi-bin/webscr for the UK