Hello Chris,
I checked and in fact the article, that would have helped, was in a beta-state and not yet public. 
I have updated it now (e.g. including your version of the error message) and made it public here:
CS53693 : Upon running FEM solution in Mechanica we get a error "Cannot find MSC/Nastran solver" check installation.
https://support.ptc.com/appserver/cs/view/solution.jsp?n=CS53693
BTW: Assuming this was in fact the missing bit of information needed, I will changed Steven's answer from Helpful to Correct, thus marking the thread as answered (which is an indicator of it's usefulness to other users).
Hope you don't mind!
Gunter