The build number is a hidden extension to the version number that indicates the build run that produced a particular release of eNICQ 6. In most cases, you will not need access to this number. In some cases, where a version is released more than once, it may matter if you've gotten the latest build of a version.
The number can be found on the properties of the executable itself. Browse to C:\Program Files (x86)\Vermont Oxford Network\eNICQ 6 (or "eNICQ 6 Test" for a test instance.) Find the executable there, named "eNICQ6" or "eNICQ6Test" depending on the mode.
You will be able to find the full version number, including the build instance, on the File version or Product version entries on the Details tab.