First check out must always be done at the package level. Single file check out and editing is not yet supported nor planned for the near future.
Modeler objects (Attribute View, Analytic View, Calculation View, Analytic Privileges, and Decision Tables) do not yet have their editors converted to run in the Project Explorer tool. You can check out these objects but you will only see the raw, underlying XML. We do NOT recommend that you edit these objects in this format. You should only use the dedicated view modeler tools. These tools currently only run via the Modeler/Navigator as they were not originally designed to edit local files but instead have an RPC connection to the HANA Backend. Eventually these editors will be rewritten to work with the Team Provider/Projects/Workspaces; but this will not happen before SPS7. So for the time being you must work in two separate workflows - the legacy one for the items listed above and the new workspace/project based approach for all other development artifacts.