The trick frankmci suggested should work fine, I use this method on my PC linking to a Dropbox resources folder. Its the textures in your projects being set to absolute paths thats stopping it from searching the additional paths you set in preferences.

If you add the Mac Dropbox textures location ...

Are the textures in the project you’re trying to relink set to relative file paths (texture.png) ?

If the textures have an absolute file path (c:/textures/texture.png) then Cinema 4D will only look in that location and ignore the additional paths set in Preferences > Files > Paths.