Answer:
You can access the subreports in the following location: (eg. for AutoPIPE Vessel version 40.0.05)
C:\Users\Public\Documents\AutoPIPE Vessel_40.0.0.5\Puser1
The word file "Design" is actually the full report. In this folder open the folder "Design" where you would find the RTF documents which are the subreports.
In order for a specific component to be opened from "Show Selected Report Calculation Details" option, the sub-document (RTF) must exist in this folder location.