Kimbakat, I read through this thread a few times to get a better grasp on your issue, but it seems that you are still a bit confused as to how to set up your DNS server properly, and why a properly working local DNS will help you in solving your problem. You mention earlier in the thread that you want your mobile accounts to work on your local network, nothing external, but then proceed to sign up for a domain on the internet. Why? What lead you to believe this was necessary?
Also, you say that none of the server manuals mention signing up for an internet domain. This is because it simply isn't required for a basic server setup. It all depends on what you want to use the server for. Further, you gave advice on the forum that likely is unecessary for many people, by leading them to believe they NEED an internet domain for mobile accounts. If they didn't know any better, this advice can lead people to wasting their money, purchasing something that probably isn't required.
And just for reference, I currently have mobile accounts running on my local network, with NO registered internet domain. Further, my server does not have DNS running, because the DNS service is provided by my internet gateway, and not any other computer on the network. Chances are, on a small business or home network, your gateway is already running DNS for you. Have you looked into this?
We have no problem helping you out with the issue, but you would do yourself a big favor by spending some time researching basic network fundamentals and planning out what you want to accomplish and what tools you will need to reach your goals. I know it doesn't sound fun, but it saves alot of headaches. On top of Jonathan's advice on checking out lynda.com, wegotserved.com has a nice tutorial on setting up OS X Lion Server on a home network, covering a wide range of topics. Also, it may be helpful to find your local friendly computer nerd (every family or company has one), and ask him/her to assist.