My IMac will not recognize my LENOVO tablet

It won't recognize an Android tablet by itself. There is an app you can add to the tablet which will enable you to access its file structure in any browser:

https://play.google.com/store/apps/details?id=com.smarterdroid.wififiletransfer

Will not 'recognize' it in what way, exactly? What are you trying to do? What did Lenovo say when you contacted them and reported the problem?