There is a Structural Modeler post in the Analysis and Design forum here:
http://communities.bentley.com/products/structural/structural_analysis___design/f/5932/t/87574.aspx
Summary is that Structural Modeler 8.11.07.94 does not work on Windows 8. It's not an officially supported OS, but maybe someone has a idea to help.
I don't believe there is any way to install (and use) older products such as Structural Modeler on Windows 8. For that matter, only our most recent release of AECOsim Building Designer was tested and certified for Windows 8, where all prior versions would work up to Windows 7 SP1.
FWIW, the same general rule applies to versions of Office as well, where the new "365" version (Office 2013) may have incompatibilities with our current Building applications, just as there were occasional issues using Structural Modeler with Office 2010. There's usually not much we can do when Microsoft shifts the rug from under our feet...