Did you mean the P-51D/H pack for FS9? I thought the true Solo P-51D (the WOP II version) comes with an FSX install version. I don't have the Solo WOP II download version so I don't know for certain but that's the way it was presented in the Aircraft Power Pack.
Anyhow, the P-51D/H pack was made for FS9 and was never updated for FSX. You won't be able to install it directly to FSX. You can, however, copy the folders from FS9 to FSX. You'll definately have to transfer the models and textures from the sim's Aircraft folder. I'm guessing you'll also have to move guages and some effects.
See this thread
I've never done this process myself; I've only read about it on various forums including this board. I believe the P-51 files sticky was added to help FS9-->FSX users so check that out as well.
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