Downloading on FPGA
iMPACT can be confusing to a new user.
Which board do you have?
What type of download cable are you using?
If the board powers-up running the built-in demo, then the clock is probably fine.
You shouldn't have to "add xilinx device". When iMPACT starts, select "create a new project" and then "configure devices using boundary-scan (JTAG)". iMPACT should automatically connect to your board, detect all the JTAG devices, and show you a diagram of the JTAG chain. It should ask you to assign your configuration file(s) to the device(s) that you want to program (or click "bypass" if you don't want to program some of them).
If you see "device ID mismatch", then the download probably failed. Clicking something to make the error message disappear probably didn't help. iMPACT should tell you the ID value it expected and the value it got. What were those values?
What happens in iMPACT when you click Debug -> Chain Integrity Test?