Most of the core packages are probably in a subfolder of /usr/local/texlive/2010/.
I'm not that familiar with mactex, but it's based on TeXlive, so you can use TeXlive utilities to find stuff.
To find where a given package is installed, you can open a terminal and type:
kpsewhich <packagename>.sty
And it should show you the full path to the file.
If you want to install custom classes and style files, I'd recommend using the TEXMFHOME tree. You can find out what this is by typing:
kpsewhich -var-value TEXMFHOME
I believe on a mac this defaults to ~/Library/texmf/. You could for example put a custom class file in ~/Library/texmf/tex/latex/myclass/ . At some point you may need to run:
mktexlsr ~/Library/texmf
to get this folder recognized by the system, but it may already be. (mktexlsr and sudo mktexlsr for system files are the equivalent of refreshing the file name database in MiKTeX.)
The command:
kpsepath tex
would show you all the folders searched for tex (.sty and .cls) files at compilation time. (If it ends in /// it'll search subfolders.)
I'm afraid I have no clue why these wouldn't show up in Mac's finder. I don't use a mac.
texhash(which for some reason they renamedmktexlsr) the local TeX folder with a sufficiently new TeX Live/MacTeX (I think 2008 or newer): it's automatically scanned on each TeX run. (The logic is that the local folder is small, so the time saving from a database is not really important.) – Joseph Wright Feb 11 '11 at 19:50