If your tablet supports OTG USB, it should start automatically. At least that's how it is with Galaxy and Nexus tablets. I know that Lenovo usually has their own GUI on top of Android so wonder if they use some custom driver or need something to be enabled in system setting? You better off searching XDA or AndroidForums for your specific tablet to post in that section.
Also, make sure you have a working cable. Can you try it with another tablet or the phone, just to be sure?
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