A “Gotcha” with SQL 2012 is that the Startup Parameters are ALWAYS greyed-out if you look for them in the usual place (IE: Configuration Manager / r-click sqlserver service / properties / advanced).
Try here … Configuration Manager / r-click sqlserver service / properties / startup parameters
dow!
Database Consultant 
All parameters are greyed-out but some one can be modified after a double-click on the parameter name. Not very intuitive…
Thanks @JBB92… Double clicking on the parameter name is the answer.
Startup parameters are still greyed out. Double clicking not changing anything. Any idea why?