Despite what the help file says, the correct value to be in the build search path field under Tools...Build Settings in the Saleslogix Application Architect is
%BASEBUILDPATH%\assemblies;%BASEBUILDPATH%\interfaces\bin;%BASEBUILDPATH%\forminterfaces\bin
not
%BASESAGEINSTALLPATH%\SalesLogix;%BASESAGEINSTALLPATH%\Platform;%BASESAGEINSTALLPATH%\SupportFiles;%BASESAGEINSTALLPATH%
Make sure you have this set or you will be unable to build you Web Platform, especially if you have any custom entities.