When you download the save editor, right click the file and click "open with" and then click Java Platform SE Binary. That should open the file in Java and the rest is straight forward. If Java doesn't appear in the menu, click "choose another app" and then "more apps" Java should be in the list.
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So from what I can tell, you don't have Java installed. You need Java in order to open .jar files, you don't need to extract it. Go ahead and grab Java and use that to open the downloaded file.
The horse cart doesn't seem to be present in 0.0.9 as it is the cause for quite a few bugs. I have experienced the issue of being unable to remove items from the crate when buying at Geoffrey's shop, too. You can take crates out of the cart, you need to be pretty patient and possibly wait for the cart to stop moving completely or find an angle where it works.
So I used the cart in 0.08 and my character now seems to have a bug. He (the player model) now stands sort of diagonally/slanted and it makes it difficult to walk up uneven survaces or steps. Another issue with this is that whenever I jump and land he begins to slide like he's on ice and has no friction, though this is fixed when I crouch and stand up again.
I'd love to get a response to this bug and maybe a fix. For now I recommend not using the cart in case this happens.