How can you view messages in the Trash bin?
Also, Is there a way that members can register their own email, using the domain extension?
(I saw there was a registration plugin, but I can not download it, the download link gives a 404 error)
What do you mean by "view messages in the Trash bin" you should be able to click on the trash folder and view any emails that are there. There is a registration plugin for hmailserver but I don't know of any registration plugins that work with any other backend.
When looking at it after login, all it shows on the left hand side is inbox folder, and a test folder I created called (other), other than those 2 folders, there is no trash folder
You need to assign the folders in Settings -> Special folders
I figured it out... Not in the way you gave, but in a different way...
Settings > Folders Tab > check the box for "Trash"
Then in going back to the main mail page, It now shows the trash folder on the left hand side.