Asset Store Tools upload?

I am trying to upload a free package to the store. I have downloaded the latest Asset Store Tools (3.2.3) My project is very simple and only contains an editor extension script. When I use the Package Manager there is an Assets folder [Select] option but no [Upload] button as mentioned in tutorials. When I Preview my project everything looks fine except there is no Package Contents section and consequently the Download link says unknown error. I haven't tried Submit yet, because it says I should be able to successfully download with the Preview option.

My folders look like this and I'm selecting the "MyFolder" item

Assets\ AssetStoreTools\ MyFolder\ Editor\ MyScript.uc Readme.txt

What am I doing wrong? Probably obvious to anyone who has published anything?

Thanks for your help.

Paul