ActiveX control not working on other computers

I have a worksheet that I was demonstrating to someone and when I had them try to use an ActiveX combo box, they were completely unable to select the box in order to make a selection. On the same sheet, there are form controls which functioned fine so I am not clear as to why the ActiveX control would not work. Both computers are on the same network and running Windows 7 / Excel 2013.

Thanks, Alan. I will try removing those files.