When working with Project Server Events, you may need to reference Microsoft.Office.Project.Server.Schema. Unfortunately, you will not find it in \Program Files\Microsoft Office Servers\12.0\Bin where you can find other common DLLs for Project Server.  For this DLL, you will need to copy it from the GAC:

1.      Open Visual Studio Command Prompt

2.      cd %windir%\assembly\GAC_MSIL\Microsoft.Office.Project.Schema\*”

3.      Copy Microsoft.Office.Project.Schema.dll to a directory outside of the GAC

4.      Go back to Visual Studio and create a reference to Microsoft.Office.Project.Schema.dll from the directory you copied it to

Just one thing to keep in mind; you may need to copy the DLL from the GAC again if a Project Server update is installed.

Hope this helps,