Since Wunderlist for Mac version 2.1.0 we have no longer supported Mac OSX Snow Leopard / 10.6.8. This wasn't an easy decision to make, but it allows us to improve performance and add more features to Wunderlist for OSX 10.7 & 10.8.
We highly recommend using one of the following options to get a hold of the latest Wunderlist features: Treat yourself to an upgrade of 10.7 Mountain Lion; use the Web App via www.wunderlist.com; try our Wunderlist for Chrome Packaged app that will give you an almost-native experience.
If you'd rather use the old legacy app you can install version 2.0.6 and click on "Never" if it asks you to upgrade:
Please remember though that Wunderlist 2.0.6 is a legacy app and is no longer supported by us, nor does it have the latest Wunderlist features.