For WIndows : 

When you download the file with Internet Explorer, it has a .zip extension, 

change that to .ipsw. Don’t unzip it, just change the extension (rename the .zip file in .ipsw).

For Mac : 

On a Mac if you read “” delete .zip.If you don’t want to change the file type to .ipsw, just use different browser such Firefox or Safari from Apple.

 iPod Touch 1G

iPod Touch 2G

iPod Touch 3G

iPod Touch 4G