The Projects->C/C++ Directories settings do not allow adding directories into the lists. Clicking on "New (Ins)" tool button doesn't add a new entry into the list. This is with build 1214.
We have some bug reports that tell us build 1214 does not save Custom settings between restarts of the IDE (something required for them to have effect.) We consider this problem critical. We intend to have a fix soon.
We are going to change our UI so a message pops up if you try to edit when Platform is anything but Custom.