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If you are referring to the P05 unit from the picture below, I can tell you that it actually does. It's not a CDU in the way you may relate it to a Flight Management System of modern planes, it's just a navigational tool called Inertial Navigation System INS.


To get it to work, you have to use the other panel.cfg coming with the 707 and you have to install the CIVA INS first.
Find the installer here. http://www.simufly.com/ins/download.html It's called CIVA INS because the guys who offer the freeware gauge called it that way.

You'll find two different panel.cfg files in the \SimObjects\Airplanes\CS_B707-300\panel folder.
One may be the one you are already using, called panel.cfg and the other one is the CIVA_panel.cfg. To activate the file with the INS included, you have to rename it to panel.cfg. That's the file name FSX is looking for when it comes to the active one.
So make a backup first and rename the files.

For a 707, the Boeing manuals stated the following genuine navigation methods.

• The classic VOR/ADF
• The LORAN system (which isn't there in FSX)
• The Doppler System (which is modelled on the 707 out of the box)
• The INS, which you can activate and use together with the other methods, or standalone

You could of course cheat and attach a freeware FMS to the plane. This would then have a CDU in the way you may refer to it, but of course wouldn't be as genuine as the other methods.

For more information on how to use the INS, look here. That's the dedicated thread for it.  http://www.captainsim.org/yabb2/YaBB.pl?num=1285100008
You'll find all the answers to common problems there, so please use it in conjunction with the search feature to find out about the rather tricky INS operation. It's worth the read and helps with the somehow cryptic stuff, confusing people at the beginning.  

If you're looking for an advanced setup in the plane, I can offer a variation here. http://www.captainsim.org/yabb2/YaBB.pl?num=1285100008#9

Be aware that, currently, the VC INS isn't fully operational, so you will have to use some 2D popups to actually operate the units. Find some screenshots in the mentioned threads.

Be aware that if you flew the old 3 crew planes with e. g. the INS for a while, the newer FMS stuff just gets boring at times. At least that's what happening here since there is so much 'auto' in the new stuff. 

But, as a side effect, one may get to know all the calculations behind this 'auto' somehow better when he's done the stuff by hand before.

Ok, I'll try that one as well, its nice getting back to the the "old school" computer navigation system, and now when I get this working, I'll be flying it as much as the 767 and NGX. 

Now you know why they write the airport's name on the terminal buildings in such big letters. 
You can see them from a distance. 

Sweet. Can't wait to try both.