XmlException importing a Vuforia AR marker

I'm absolutely baffled. Using my 2011 MBP running 10.7.5 and Unity 4.5.5f1 I'm able to successfully use the Vuforia Unity Extension 3.0.9 for Android and iOS, with a target marker I downloaded from the same website.

When I use the same version of Unity, the same version of the Vuforia Unity Extension and the same target marker unitypackage on a 2011 iMac running 10.6.8 I get an XmlException: invalid data whenever I import the target marker.

I can't find out what the error is and I've examined the Xml file in Assets/StreaimgAssets/QCAR/.xml and it's character for character identical to the file on my MBP.

For long and complex reasons I'm not able to upgrade the iMacs (x21, I work in a school) up to 10.7 - I'm hoping it's not simply an incompatibility (though it's an Apple, so I know it's far from impossible).

Any ideas?

Many thanks!