Updated April 2017:
In some situations a series of VBA errors can occur when starting AECOsim Building Designer. Two possible causes are:
In the second case we have a number of support articles detailing the actions to take to resolve these problems.
However before delving into these solutions an easier option is (assuming it is installed, and it needs to be to take advantage of the DataGroup 'Edit in Excel' feature) to run a repair installation on Microsoft Office. This very often resets the VB/VBA components to the required state.
For background information and full details read Microsoft Security Bulletin MS12-027 disables VBA Scripts (updated 04-20-2012)
To resolve this run Microsoft Fix it 50864
For background information and full details read Microsoft Security Bulletin MS12-060 (update to MS12-027) disables VBA Scripts (released 08-14-2012)
To resolve this for Windows 8 based systems run Microsoft Fix it 20097
To resolve this for Windows 7 or earlier based systems run Microsoft Fix it 50930
[Note the Microsoft 'Fix It' links are correct at time of writing but are external links so may change]
Microsoft's April Security Updates Kill VBA Scripts
VBA errors when using certain Structural or Mechanical tools
Class Factory VBA error opening AECOsim
VBA Error - Class not registered deals with unregisterd fm20.dll module (added April 2017)
If it is related to an installation/upgrade/downgrade of Microsoft Office, it may be that FM20.dll is not in the registry. In my situation the file was intact and just needed to be registered. I have 64-bit architecture. I ran cmd.exe in administrator mode and executed:
for 32-bit I think it would be
This worked for me but I'd exercise caution when dealing with the Registry.