Processing order: everything gets processed equally

GTD dictates that you should process the top item first. There are no items more important that other when it comes to process them. When you are processing you are not actually doing anything but deciding what to do with each item.

So in zendone you can't select which item you are going to process next. The processing panel will always contain the oldest element pending to be processed.

everything gets processed equally

David Allen, Getting Things Done: The Art of Stress-Free Productivity

Ok, but I really want to select which item to process next

A workaround is to just search the title you want to process in the search box on the header. That will hide the rest of inbox items and will let you process it. But remember, we didn't tell you that!