fix problem with Archiving
The solution that worked for me was to add
  $(SRCROOT)/../vendor/BoxSDK  recursive
to
  Header Search Paths->Release
In Build Settings of FolderPicker-SampleApp

but this is a bad solution and it would be better if it was somehow fixed in BoxSDK
see also
http://stackoverflow.com/questions/5543854/xcode-4-cant-locate-public-header-files-from-static-library-dependancy/10159481#10159481
for a different solution that for some reason did not work for me