I know there have been problems in the past with installing products into newer versions of a flightsimulator.
So here's a handy toolkit from VisualFlight. It will make all your old FS2004 products [e.g project AI, PSS A320, PMDG] see FSX as the directory for FS2004 without messing up your directory of FSX.
Here's the link: http://www.visualflight.co.uk/fs2004toolkit/
Once you've downloaded the file, put the "FS2004_is_Here" in your FSX directory and double click on it. There you go all is done, now you can install all your old PAI traffic into FSX without a glitch.
Hope it helps
P.S: Drop a word of advice on which add-ons worked and didn't work for you while using this tool.