Firstly, you need to download the SimuFly Delco CIV-A INS. The latest/current version is 1.60 (32bit).

After you have installed it, you have to go to FSX\SimObjects\Airplanes\CS_B707-300\panel folder, and rename, or backup, the panel.cfg (I renamed mine to panel-orig.cfg), then rename the CIVA_panel.cfg to panel.cfg.
Once that's done, start FSX/SE, select the CS 707, press F10 (the default setting for the 2D Cockpit), as this activates the INS for use, then return to the VC (F9), and start using the INS.


For all information required about the CIVA INS in the 707, see this topic:
CIVA INS - THE ONLY THREAD PLEASE