Thanks for the reply.
Did you create the record set to lookup the tax rate form the database? 
Yes. I created a database with 3 columns; TaxId,city and tax.
I also created a record set with new connection and filtering by city. I tested and it shows correctly the tax depending on city.
Please see the screen shot of tax's database.
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']'
In my page I don't have the "Based on carts taxable subtotal". I have and see only 2 options:
Base on the cart's taxable total and Based on the cart subtotal.
To be honest I don't know what to put in Cart subtotal "times" field.
I attach a screen shot to see my set up:
