After waiting a long time for this answer, I have sorted things out, and I think that many other people will need a simple solution for this question.
Warning: This solution is for the eclipse user who uses Indigo or lower version. Still looking for solutions users who use other IDE’s for android.
When extending the FragmentActivity for your application with version of 2.2 or lower, you can download the android.supportv4.jar from here or you can get this from your android-sdk directory ..\android-sdk\extras\android\support\v4 and put into your project.
If there are no directory libs in your project, create libs directory and paste the android.supportv4.jar file in this directory.
From eclipse project workspace:
Select project/application in which you want to use. Right click on Project and select Properties option. In this select Java build path and select the tab Libraries
Now click on Add Jar. It will display current list of projects with selected current project.
Expand this and go to the libs directory and select android.supportv4.jar file then click ok. It will display in list box now. Remember add the jar file as relative path path not as absolute path, so whenever if you change the directory of your project or open in another machine then will detect automatically for the project directory. Now click on Ok to close the Properties dialog.
You are able to import the android.support.v4.app.FragmentActivity; by placing mouse over the FragmentActivity or pressing the ctrl+shift+o to import missing files.
Hope you can got now FragmentActivity class.
If you are using the eclipse juno, then no need to worry for downloading the support jar file. It will automatically put into the libs directory and automatically added to your project path. If in case not then go through the Properties option and add it.
One more thing If you need to support your application for lower version then build with the higher version and add your this line into your androidmanifest.xml file
<uses-sdk
android:minSdkVersion="3"
android:targetSdkVersion="15" />
this will now support for 1.5 to 4.0.1 version devices.