System.DirectoryServices Not Found

Hi, I have installed the TimeTracker app in my system. Its works well. I created a local project for this TimeTrack application to test it locally. And when i am building the application i am getting the followingerror; Namespace or type 'DirectoryServices' for the Imports 'System.DirectoryServices' cannot be found. I could'nt find the System.DirectoryServices in my VS .NET. How can i solve this? Did any one of you find this error?. With Thanxs, Bagavathy.