USB microscope not showing plant cells?

I recently bought a USB microscope (Veho make) and it says that it can zoom in up to 400x. When I am looking at, for instance a leaf under the microscope, I am not able to see any cells. I am wondering if I am using the microscope properly, and if I am not could you please tell me what I am doing wrong. I am just looking through it and adjusting the zoom but it is not zooming in any where near 400x. The microscope is a Veho VMS 004. Thanks very much for any answers!