The WYSIWYG editor is one of the default plugins that ship with Q2A. No extra installation required. It should also be the default editor, if I'm not mistaken. Otherwise you can change and configure editors under Administration center → Posting.
Edit: If for some reason you modified the WYSIWYG editor plugin (which your comment below seems to indicate), you can restore the original plugin like this:
- Download the Q2A v1.8.8 archive again (if you haven't kept it around).
- Delete the modified editor plugin directory from your Q2A plugin directory.
- Extract the contents of the archive somewhere.
- Copy the editor plugin directory from the extracted files to your Q2A plugin directory.