symbolic link not created
As Vincent points out, the proper command is ln -s, not ls -s. In addition to that, ln will throw an error if the target of the link (in this case, the Mydocs folder in Dropbox) already exists.

The proper way to do this is to delete /Users/Joe/Dropbox/Mydocs, then run ln -s /Users/Joe/Documents/Mydocs /Users/Joe/Dropbox/Mydocs.