If redsn0w can recognise it then, for whatever reason, it is corrupt or invalid. IE it is not the right file or it has not downloaded properly.
Your best source for IPSW files is Apple. There is always a post on redmondpie which gives you direct download links for each iDevice.
The 4.3.1 page is Download iOS 4.3.1 for iPhone 4, 3GS, iPad 2, iPad, iPod touch [Direct Links] | Redmond Pie
If you download the IPSW using the iPad1 link on this post you should be good to go.