DarkSend is just a little checkbox on the right side of the wallet under the send tab.  It requires the use of the beta wallet.  We now are on RC1 and in case that's not the version you have, you can get it here:

https://darkcointalk.org/threads/darksend-rc1-official-release.274/page-2

Open the link to a PDF file, then download from the links at the bottom of the last page