I have installed the INS according to instructions in Tips and Tricks #496.  The INS installed on all versions of the 707 but I can't get it to turn on.  The Operator's Manual for the INS refers to a Mode Selector Unit (MSU) to start and align the INS, but I can't find it anywhere in the cockpit.  

Can someone tell me where it is, or is there some other means to toggle the INS on and off?

I found the answer to my own question.  In the overhead view in the corner I found the MSUs for each of the 3 INS CDU heads.  I was able to turn on the 3 CDUs.

However, now I can't get the CDUs to accept a flight plan.  Of the 3, the one in the middle shows the 707's present position, but if I click forward to other legs of the FS flight plan there are no coordinates shown.  Also, CDUs #1 and #3 only show all zeros in the present position, and also don't show any additional legs of the flight plan.
I must be missing a step in the initialization of the INS, can someone tell me what I'm doing wrong?

Also, does the INS add-on require FSUIC?  If so, which version is compatible with it and with FS2004?

I found the answer to my FSUIPC question and downloaded the .dll for FS2004.  The INS is now responding better than before, but I still can't get it to accept a flight plan.

I'll welcome any guidance.

I'm not sure what you mean by "accept a flight plan"? AFAIK you must:
1. enter your co-ordinates manually for each waypoint, or
2. Use the ADEU card reader after converting a saved FS9 "PLN" file to ADEU cards with "FPConverter.exe" (freeware).