@kmsravindra I've added plugins manually with Phonegap CLI, but it should be the same for Cordova
The important bit is to run the command from the correct directory...took me a while to figure this out as the directory structure is quite confusing on a CLI project.
Open your app folder at the command prompt, e.g. /platform/iOS/yourAppName/
At the command prompt type: phonegap plugin add cc.fovea.plugins.inapppurchase
You may need to run: cordova plugin add cc.fovea.plugins.inapppurchase
And it should add the plugin correctly... hope it works.