Herman,

to modify core modules, you ideally only need to install source package like install using the installation wizard, e.g. from "install new module" in Host :: Module Definitions menu item. If this doesn't work, I suggest to install runtime version and unzip source package directly into /desktopModules/<moduleName> folder. However, if you are using DNN 4.x.x and VS 2005, you can edit native DNN4 modules only  - unfortunately many modules are still DNN3 modules (even in the core, e.g. Forums or UDT), that can run in DNN4, but not be modified without migration. I.e. you either need VS 2003 and DNN 3 to edit DNN3 modules directly or you need to migrate them to DNN4, preferable using the DNN migration wizard, and you can edit them with VS2005 and DNN4. HTH