While I've used SolidWorks for years, I've never really had the opportunity to use flexible subassemblies before.  So when I went to set a subassembly as flexible, I can't?  The SW help tells me to do this:

Making a Subassembly Flexible

To make a subassembly flexible:

1. Right-click a subassembly in the FeatureManager design tree and select Component Properties. 
2. Under Solve as, select Flexible, then click OK.
   Flexible is not available for lightweight subassemblies.

   In the FeatureManager design tree, the icon changes to to indicate that the subassembly is flexible.