You add DLLs references through your project's References. In the solution explorer, you right click References and click Add. Locate the DLL and select it. Now that namespace and any classes in it can be instantiated (well, public ones and such).
You can also put all the projects together in one solution. If you have one project open, you can right click the solution node (the topmost entry in the solution explorer) and click Add. You can then add a new project or an existing one. Once added, go to the references and click add but this time click on the Project tab - it will show all the projects in the current solution. Now that namespace and any classes in it can be instantiated (well, public ones and such).
-Nerseus