Sigma is a friendly little web applications who has been tasked with making it easier for a firewall administrator to create large numbers of 'MIP entries' without having to key them all into the GUI/CLI one by one.
Once you provide Sigma with data, he'll burn some CPU and then provide the ScreenOS commands for you to create as many MIP entries as you need.
To make Sigma work, you must provide a tab delimited list detailing the NAT IP and the Native IP. Also, you must tell Sigma the gateway interface that will receive NAT'd packets on and the virtual router that'll process the flow. The fields are already populated with sensible defaults that work well, so simply click submit to get a feel for how the input influences the output. You might want to check the ScreenOS CLI manual if you're don't immediately grok what you're looking at.
Sigma is there as a convenience to you, nothing more. If you choose to use Sigma's output on a device, it's your responsibility to ensure the correctness of the output. Sigma and his author can accept no responsibility for any loss or damages that may occure.