Try to run the game with administrator privileges. Selecting a file and uploading it to the steam workshop requires the game to fiddle around with temp folders and temp files.

I already run the game as administrator.

Do you get any kind of error message?
Otherwise the process is this:
Be online on Steam while starting the map editor.
Select the type of file you want to upload.
Browse after the file. If it's a map it's usually in your custom maps folder.
Select the file and preview the information to be uploaded.
Press the submit button and let it upload the file