When I run my Today Extension on the simulator everything works fine and it displays the following as expected:
Few points to remember while using app extensions
Bundle Identifier for the target extension should be com.companyName.AppName.ExtensionName
You need to have separate AppID for the target extension, with identifier specified as com.companyName.AppName.ExtensionName and create provisioning profile with this AppID.
Also, the appGroupID created should be embedded in both the AppIDs(for application as well as extension).