dataitem.h:64: error: ISO C++ forbids declaration of 'SimpleTimer' with no type
The first error is the important one. This looks like the compiler cannot find a library and the object declared cannot be defined. Does the software you are compiling use an additional library?
Libraries that are not distributed with the Arduino release should be installed in a 'libraries' subfolder of the sketchbook folder. This makes it easy to have them remain the same when you upgrade the versions of Arduino IDE (ie, no need to reinstall).