Now that you have your new drive tray and new servo arms in place, you need a controller and remote. You could use a commercial RC hobby controller that has the ability to mix mecanum wheels. However, depending on your goals, you might miss out on some expandable options that can really only be done with a microcontroller such as controlling screens (as in the Omnibot Eyes project - yet to come).
As a kid I dreamed of getting one of the Omnibots from RadioShack! Each Christmas and birthday I would beg and bargain for one but to no avail. They were too expensive for a toy, my parents would say. I dreamed of "programming" it to do all kinds of things little knowing that my parents were right. As cool as they were, Omnibots just weren't as useful as the advertising and my imagination made them. Little did I know that they couldn't even move their arms! Let's fix that!
In this project, we'll walk through installing the mounts and making the necessary modifications to the arms of your Omnibot 5402 or Sr for motor control of the arms. My goal was to create a structure to move the arms by the shoulder with minimal modification to the bot itself. It will still require two parts being permanently modified but it could still be reverted back to a cosmetically but not functionally original state.