In the Material Studio is written rSCAN functional doe not support any form of Dispersion correction.

In the setup dialog, it is possible to switch for rSCAN dispersion correction on and activate TS dispersion correction. This is probably a bug - the correction is not applied (nor supported in CASTEP) and this menu behaviour is an error.

On the other hand following article (10.1021/acs.jctc.7b01172) illustrates support of MDB* dispersion correction in CASTEP. Manual editing of the .param file makes MDB* launching in material Studio CASTEP possible.

I believe it will be worth to correct the manual, correct the setup menu and add MDB* activation in the menu not to be forced to do the work manually.