Actually we have a few things to help get you started...the links I'm about to mention are all found here...(links are in blue)
Programming Your Uniden Scanner - The RadioReference Wiki
a. Let's start with making the connection between the PC and the scanner. You need to load up drivers and set up the 396xt's port to make this happen. The 'Connecting scanners via USB' article will help - you need to use Windows Device Manager to figure out what COM port the driver allocated. This is the same port you need to use in ARC-XT
b. Don't even think about using the Uniden manual to understand the radio - the 'Easier to Read' manual is far and away better. Click on the link to get to the site homepage and select your model from the list. It also has information on how to set up the port on the 396XT...
c. Finally the Uniden DMA FAQ covers a lot of topics - including many not addressed by the Easier to Read guides.
Before you start programming, you should take the time to understand how DMA works - there are no banks - it's all systems, sites and groups. The Guide and the FAQ together will get you going. There are Excel based worksheets floating around - perhaps someone can send you one...
You might want to check on our Mass. forum and/or the Scan New England
website and see if there's a MSP file for ARC-XT someone is willing to share with you. Sometimes having a working example makes the light bulb turn on.
HTH...Mike