Are you talking about registering a string with Autodesk to prevent naming conflicts?
That's something you should only need to do if you are releasing a product for commercial sale. Registering a string with Autodesk means that you should not have naming conflicts with other third-party products that also use registered symbols.
The string would be added to anything your application creates in a global namespace. For example, if you wanted to create a RESET command, and you registered MMMM as a registered symbol, you could create a command named MMMMRESET and you would know that no other product will create a command with the same name. Similarly, if your application creates any custom entities, those custom entities would have names that start with your registered symbol.
If you are not creating something for retail sale, there should be no reason to need a registered symbol.
Not sure what you mean about not being able to use the Autodesk SDK... Are you talking about the ObjectARX SDK? What exactly in it can't you use? And are you aware that there are free IDE's available, such as Visual Studio Express and SharpDevelop?
