Unable to enter in Programming mode (AT_Studio)
In most cases it is one of the following problems:


Clock speed is set too high in the IDE.  The microcontroller you are attempting to communicate with is running at a lower clock frequency and cannot keep up.  Set the IDE's clock rate to the lowest setting and try again.
There may be a wiring fault with the product you are connecting to.  This is very common in hobby projects as opposed to manufactured products.  Done it myself more times than I can remember :)  Check that you have a pull-up resistor (4.7k is about right) between your nRESET pin and VCC, as that is a common omission.
There may be a firmware incompatibility on the programming tool you are using.  Your programming tool may have older firmware than the IDE can use.  Atmel's IDE will usually attempt to update the tool's firmware for you.  
The computer may have the wrong hardware driver installed for your programming tool or has misidentified the tool.  Uninstall the tool and attempt to reinstall using the latest drivers from the manufacturer.