I wished I had it as well at first, but found that I don't really need it and it is really a huge slowdown for me.
1. Once making a Visual Studio 2008 website project out of the 5.1.2 install files and running the install process, this is what I did.
1. Run DNN from inside Visual Studio. Right click default.aspx and choose view in browser.
2. Once loaded, click login and sign in with host username/password.
3. Click Host, then click module definitions.
4. At the bottom of the list of installed modules, you will find a create module button. Click it.
5. Click add new definition button/link. Enter the information that is requested by the new definition section and click create/add definition button. The choose a definition to edit combobox will auto populate with the new module definition.
6. Continue filling out the create module wizard pages.
7. When done with the new module wizard, you can add module controls through the same wizard. Just go to Host>Module definitions and click the edit this module link next to the module you want to edit.
8. Create the module code. Continue like normal and create all of the module code/controls importing module controls as needed.
9. The module definition file doesn't do anything in a visual studio project and is only needed when you create a package for install on another dnn copy.