Hello @sutter_zhou,
Sorry, my bad! All the MBDT toolboxes are installed in the MATLAB AddOns specific folder, which may differ for each user (everyone can select his own). To find your own or to change the default one, go to MATLAB -> Preferences and under Add-Ons you may find the toolbox location.

But, once the I.MXRT toolbox has been installed, you can type the following command in MATLAB's Command window: cd(mbd_find_imxrt_root) and this will change the Current Workspace to the one from the image.
Hope this helps,
Marius