hi Nilesh,
Mage stands for Manifest Generation and Editing Tool, and you can find this in VS2008, Windows SDK, and .NET Framework 2.0 SDK. you can use this to create application and deployment manifests if you want to deploy your application. if you use C# or VB, the ClickOnce publish wizard creates the manifests during publish, so you don't have to manually create these.
for more information, see Manifest Generation and Editing Tool (Mage.exe)and Walkthrough: Manually Deploying a ClickOnce Application.
m.