Software giveaway sites like Giveaway of the Day and BitsDuJour feature MixPad Professional once or twice a year. They offer a for MixPad Professional. You have to install the software on the specific day of the giveaway, but you keep the key forever. This is the only legal "free code" source.
Keep in mind that these alternatives might have different features and user interfaces compared to MixPad. free registration code for mixpad