how do i get authorize.net and ecartsoft to work together?

I am guessing you might be referring to http://www.ecartsoft.com/?  I could not get the website to load for me.  Usually a shopping cart will have some type of 'add-on' or mod for the electronic payment gateway.   And with Authorize.net, they will give you a 'key' to enter into this to help process the transaction.  
(Maybe if you can confirm this is the URL?)

https://www.ecartsoft.com/cgi-bin/home.cgi would, i guess, be the "direct" url to get to the home page of ecartsoft.

there is a section that allows me to configure the payment gateway, but there is no where in that section that allows me to enter in a key. (please see the screenshots of the page)

first part of the page : http://img15.imageshack.us/img15/2572/pt1.jpg
second part of the page : http://img135.imageshack.us/img135/3135/pt2.jpg

both ecartsoft and authorize.net are in test mode - and both site says to leave it in test mode until i'm sure that i'm ready to launch

corey - thank you for your quick turnaround, you don't know how much i appreciate it, i will give your tip a go and return back to you with a fail or win.

thanks!