I am using SolidWorks 2014 SP2 and I am practising for the CSWP exam part 3 which deals with assemblies.

I need to get a robot arm to rotate and extend with a pneumatic cylinder.  I can get the different sub assemblies to move as required but when I assemble everything it all locks up.  Okay, so I should set the assemblies to flexible, I know that, but the check box is constantly greyed out making it inaccessible and nothing I try works to resolve this.  Any ideas would be gratefully received.

Andy