You should not have created any new RuleSets when you built this application. You should have built the application in the existing RuleSets. What is the name of the RuleSets your application is built in? What is the name of the classes. Generally you can rename the classes on the imported files, then they can be found and will work.
I have had people be successful with this exercise by changing the names of the imported files using the refactoring tools. They changed the imported rules to match thier naming constructs and were able to do this exercise. I am not sure why you are experiencing something different.
I think the reason might be that while importing file, no rulesets are being populated in the tool where i can refactor ruleset names alongwith rule class names.
Any idea why this might be happening or should I exexute import in some specific sequence?
I am not sure why this is not importing correctly. You can move ahead and finish additional exercises without this one working. Once you have completed the course, have us reset your exercise system and rebuild this again using the correct class structure. You would not have to build the entire exercise again. We could do the reset now if you like. When you want us to reset your exercise system send an email with the your name and course to SelfStudySupport@pega.com.