How can I make a soft link in Lion?

You can create soft links (a.k.a. symbolic links or symlinks) from the terminal, the same way as in any other Unix system.

I'm not familiar with Dropbox so I'll make up an example assuming that there is a folder Dropbox in your home directory:

cd ~
ln -s ~/Dropbox/aFolder Desktop/aFolder

Note that ln takes the link to create as its second argument; think of it like cp.

Also, a warning: if you create a soft link with a relative path, for example

cd ~/Desktop
ln -s ../Dropbox/aFolder aFolder

then the link will be relative to its location at the moment, not to the current directory when you created it! So, for example, if you moved it into ~/Desktop/Folders then it would be pointing at ~/Desktop/Dropbox instead of ~/Dropbox.

You must use the Unix ln command from Terminal.

ln -s existingname newname