I strongly suspect you are on R2018b and am posting this. I suspect the MKRMotorCarrier is not showing up due to a bug that a user identified. The easiest way to confirm is Click on Add-Ons > Manage Add-Ons on the toolstrip at the top of MATLAB. Here check the version of Arduino_Engineering_Kit_Hardware_Support_18b. This is the file you must have for R2018b, if you have Arduino_Engineering_Kit_Hardware_Support uninstall that and install the latest version (1.1.1) of Arduino_Engineering_Kit_Hardware_Support_18b and you will get MKRMotorCarrier Library. You don't need the ArduinoSensors library in R2018b.
If you are on R2018a, please uninstall and reinstall the Arduino_Engineering_Kit_Hardware_Support toolbox from Add-Ons > Manage Add-Ons.
Does your Simulink support package for Arduino look something like the following image? If not, please uninstall and reinstall this as well.
If you decide to uninstall and reinstall any of these, please remember to open MATLAB with administrator access as that helps. Right click on MATLAB icon and select Run as administrator for this.