Compatibility problem with .xcdatamodeld between Xcode 4.1 (Lion) and Xcode 3.2.5 (snow leopard)
The format of .xcdatamodel bundles was changed between these 2 versions of Xcode. The format of the elements and layout are now xml based, whereas they used to be binary data.
You can change the minimum required Xcode version in the file inspector for your .xcdatamodel file.
XCode 4.1 is available for Snow Leopard. I suggest you just try using the same version on both machines.
It is more reliable to use latest XCODE ver 4.2. Compatible with lion (OSX 10.7).There are amazing features like ARC,storyboard and much more.