Did you create the record set to lookup the tax rate form the database?
Is it setting the tax rate in the $_SESSION['taxRate'] correctly?
Add the following to your confirm page to see if it is being created gorrectly:
echo "tax rate: ".$_SESSION['taxRate'];
if the recordset is working properly, this should show the tax rate.
In the screen shot, you have not entered a name for the session variable, nor have you entered anything to multiply the taxable total by.
For the trigger use:
session variable exists:
if the session variable 'taxRate' exists.
for the calculation use Based on carts taxable subtotal:
Taxable subtotal 'Times' '$_SESSION['taxRate']'