If you go to the page where the sessions are first created and double click on them on in the server behaviour panel then it then click OK on the box that appears it should the recreate them in the bindings panel for you to use...
...you can do this also if you created some other webassist behaviours such as the html editor and if you were to delete the ckeditor just double click on the server behaviour in the page that its code is first used and the directory structure will be recreated (as an example)
Chris