You need to copy ALL the "UserData#" folders (where # is the number of the neighborhood, use just "UserData" for Neighborhood 1) to the game's directory on the Mac.
For the downloads, you'll need to copy the following directories, all located in your Sims' installation folder (usually "C:\Program Files\Maxis\The Sims" on the PC):
\GameData\Objects (NOT Objects.far)!
It's best to have the same expansion packs on both the PC and Mac, otherwise you should reinstall downloads to avoid conflicts.
You should not need to convert any files. Only the filenames of Skins may get too long so they become unusable on some versions of Mac OS. You can find tools to solve that problem at TheSimsWorkshop