how to access different directories outside cygwin home dir

You can access the root of your C: drive, for example, from cygwin by specifying the directory /cygdrive/c, e.g.,

$ cd /cygdrive/c

Similarly, the path to any file on your C: drive can be constructed by prefixing the Windows path (without the "C:" and with \s changed to /s) with /cygdrive/c, so C:\Dropbox\myfile would be accessed from within Cygwin as /cygdrive/c/Dropbox/myfile.

If you really wanted it available in Cygwin, besides getting it directly (as @garyjohn's correct answer) you can also try to use the mount command to map the path to your home dir. Something like this may work.

mount C:\dropbox\cygwin /home/me/dropbox

I say may, because though I'm pretty sure it will work, I have no Cygwin install to test it, and I'm not sure if Cygwin will refuse to create it because of filesystem loops (it should allow it)

As @starcorn mentions above, ln works great for this.

$ ln -sv /cygdrive/c/Users/JohnWagg/Dropbox/ ~/dropbox
`/home/JohnWagg/dropbox' -> `/cygdrive/c/Users/JohnWagg/Dropbox/'