How to define folder in which all the project-files are stored?

I did a normal installation under MAC OS 1.8
Now I would like to define the folder in which the project-files are stored in my DropBox-folder, so that I can also access the projektfiles from my laptop.
Is that possible?

The setting of the wwwroot-dir in the settings-dialog did nothing as far as I can see.

Thx for a short answer.