500mm Telephoto Lens for Nikon D3100?

Hi, I bought this item from ebay and just received it. I have a problem using this lens. I tried taking pictures without flash outdoor and it comes out to be dark. Outside right now its cloudy. I tried taking with flash and it comes out to be too bright. This lens must be on manual mode. I tried every single iso setting and it is either too bright with flash or too dark without flash. Anyone please help me or tell me what setting I should put on. I need help with this lens. Is this lens good for outdoor like the beach in a sunny day? Because I can't seem to find the right mode or know how to use this correctly!

You really need to take a class in basic photography.

* Using a flash with a 500 mm lens is problematic. * at the focus point, usually over 100 feet, the flash will provide too much light in the foreground (look up the term, "inverse square law". * unless the lens is electronically connected to your camera, metering is impossible (you will have to buy an external meter) * Shooting during the day, just use the Sunny 16 rule. 1/ISO @ f/16. Since your 500 mm is probably a f/8 mirror lens, that will mean your exposure will be 1/ISO x 4 @ f/8