The FSDT loads perfectly with no errors or anything
This means everything is working, otherwise you wouldn't see any scenery.
but I cannot activate the sceneries.
This is not very clear. Please indicate exactly what the problem is. Do you have errors ?
The couatl engine is in fsx, but it never ask me to run the program like fsx should.
If with "Couatl engine is in FSX", you mean you SEE the "Couatl powered products" menu under the FSX "Addons" menu, it means is running correctly and there's no problem with it. In any case, since you said "FSDT loads perfectly", I assume you see the scenery correctly, which just confirms it.
If you meant something else, please clarify.
I have tried the sim connect thing, and I have gotten any errors
This is as unclear and generic as it gets. Please indicate what errors you got, and we'll see if they have anything to do with this or FSDT.
The REGEDIT didn't seem to work neither.
Without a clear description of the "can't register" problem, it's not possible to say if this has anything to do with it. There's no need whatsoever to use REGEDIT, you might have confused with a suggestion to another user that used PREPAR3D and the problem was caused by the migration tool. You said you used FSX, so this shouldn't be relevant to you.
The latest FSUIPC is downloaded also.
None of our products requires, uses, comes with or has any interaction with FSUIPC.
So if I could just get the add-on Manager to load it would make my day.
You said "FSDT loads perfectly" and you said (If I've understood correctly) you have the Couatl menu in FSX. Both things were not possible to have or see if the Addon Manager wasn't loaded so no, unless you meant something different, it's not possible you have the sceneries working and Couatl working, but no Addon Manager.
If you meant something different, please clarify.