Dynamic memory allocation problem

I have a simulink model in which I need to use dynamic memory allocation ro run the model.

For some reason, in the config. dialog box the "dynamic memory allocation for MATLAB functions" is grayed out and I cannot enable it.

Any idea is appreciated.

[Nils Meyer]

had the same problem. installed simulink coder and matlab coder. then in "code generation" below "simulink target" i selected grt (generic real time target). now it works