I admit to being a bit confused by your post; I have never ‘exported’ a file from WavePad – I just save them into folders. (‘Save File As’)
Here is how I would recommend performing your task...
Once the file has been edited, save it with a name and file type (mp3). If there are multiple files, I would create a new folder (on your desktop, or wherever you prefer to save such things)
To upload to dropbox: Open your Dropbox. Select – and Copy - the files/folder you wish to share/upload – and then Paste onto your dropbox. Fairly simple procedure; you are uploading files – not WavePad projects.
Hope that helps?
Also, regarding sharing dropbox folders/files. I'm unclear about what you mean by sending it to a different dropbox, but this is how I usually share dropbox files.
With your Dropbox window open, Right click on the file/folder - this opens a list - select 'Copy Dropbox link.' Now you can paste this link wherever you wish. For instance, in a facebook message, or in an email.