ARC250 definitely makes this job far easier than by entering things manually. Re the rest of your question, let's start with the basics - what is it that you really want to listen to, and what sort of documentation do you have on their radio systems? The Radio Reference Database information is a great start at getting the information you need. If you can describe what it is that you want to listen to (Plano Fire, Dallas Police, etc.) I'll bet there are some folks here who can help provide you with some sample .mem files that you could download. Given a few templates, it shouldn't be hard to edit those to fit your specific needs.
By far the hardest part of the setup is if you have to configure for a new trunking system. Once that part is done, it's usually very easy to add additional channels. But before we can tell you how to do that, we will need to know which cities you are interested in listening to. From that, we can look up information on the types of radio systems they use and get a better sense of what other information you will need.
