From what I've gathered from the help page you linked to and from the DFP Product Help Forum, you must first contact DFP/Google and manually request that they turn on this "3rd-Party Services" menu item so that you can select it and link your Adsense account.
The help article you linked to reads:
"Ask your account manager to enable your DFP network for dynamic allocation. This ensures that the "3rd party services" section shows up in your DFP account so that you can complete the following instructions."
Here are two posts that briefly point this out. I guess your best bet is to click on the CONTACT US link in the upper-right corner of the DFP help article you linked, select AdSense and the next logical menu items, then a form will appear, through which you can send Google your request to turn on the '3rd-party Services' link.
I have no idea how responsive they are to these manual requests, but this is the best I could come up with.