The Electrical help is not included when choosing Help > AECOsim Building Designer Help from within AECOsim Building Designer SELECTseries5. The only way to load the Electrical help is to directly launch (double-click) the file itself found under C:\Program Files (x86)\Bentley\AECOsimBuildingDesigner V8i Ss5\Documentation\ElectricalSystem.chm.
If you simply rename the file from "ElectricalSystem.chm" (singular) to "ElectricalSystems.chm" (plural) that should allow you to open the Electrical Help directly from within AECOsim Building Designer SELECTseries5. This has been logged as Defect 91657 for resolution in a future release of AECOsim Building Designer.