Hagar, I think I may have installed an update. But I sort of interpreted the file description as being the full package. It sits on page 3 of Military Aircraft and also appears on Simviation's home page under 'News/Hot Picks. It suggest that it's compatible for FS9 but also says it will work with FSX!
Right. Just as well I asked for confirmatiom. I downloaded P-3C_V29.zip posted
HERE to check it out. I wouldn't normally try an FSX addon in FS9 but, as you point out, the file description suggests it's suitable for both. It's not clear how it's been updated for FSX.
On extracting the file I found another zipfile inside it. Extracting this one revealed a folder named P-3C_V29 containing the complete aircraft & not the auto-install file I was half-expecting to find. I opened the Panel subfolder to look for those zipfiles you mentioned. These are gauges in CAB format as I suspected & mentioned in my first reply. They will function just fine left right where they are. Alternatively you can paste them along with F16.gau into the FS9\Gauges folder.
CAB files will look different depending on your software & which program they're associated with. As I have WinZip installed they look just like zipfiles. (See my shot).

I installed the P-3 to FS9 by pasting the P-3C_V29 folder to FS9\Aircraft. I don't know a lot about autopilots or radio but everything seems to work as advertised. In fact it's a very nice aircraft & I enjoyed testing it. It also has a very nice DVC. I suspect the reason you didn't see it is that there is at least one variation of each model without a DVC.
I then tried it in the FSX Demo. I note from the Readme that it's not fully FSX-compatible & has some known issues. It's not clear exactly what these are as half of it is in Japanese. I was expecting problems with the F16.gau on the panel as this hasn't worked on other FSX updates I've tried but this version seems OK. The DVC also appears to work fine. (Make sure the "Show all variations" option is checked on the FSX selection menu so you can select the versions with a DVC.)
I have no idea why you should be having problems with this aircraft. It's possible the gauges won't display properly on certain video cards or graphics settings. Make sure you have no earlier versions of this aircraft installed that could cause a conflict.