I believe it wants to install to the Documents Folder (it's been awhile, so I'm working off memory here). Documents would be a standard folder in MacOSX. Is it possible you do not have a Documents Folder (it would be in your user Folder)?
Where do you download files to? Will it install in that location? Do you use anti-virus software (I haven't used any for years and years, but it's worth asking)? Are you saying that the installer can't find anything it will agree to install in?
Are you trying to install the complete game or just the patch? If the latter, try downloading the complete game.
By the way, I should have mentioned that any work-around you try, you should de-license first before you move files around (if it turns out that something works).
Can you post a screen shot of what your screen looks like with the installer running?
I'm just throwing out ideas here.
I think the problem you are having is very localized, which makes it tough to diagnose. I have OOTP functions (not license related) that have never worked on my Mac, yet do work for others -- and I can't figure out any reason for it, even though I've tried lots of experiments. Nor do I expect that anyone else will ever figure out the reason.