Described problem can be solved by the following steps:
-In visual studio right-click your installer project and select view->custom actions
-Right click the custom action for the phase(install, uninstall..l) in which you need to access the source directory of your msi project and select properties
-Enter the following line under CustomActionData:
/sourcedir="[SOURCEDIR]\"
Now the source dir in your custom action can be accessed by the following line:
Context.Parameters["sourcedir"]