If the Genius Bar person showed you by using the App Store and your MBP came preinstalled with Lion, they showed you incorrectly.
If you restart your system using Command+R+Option keys and keep them pressed down until you see a white screen with a spinning globe, you will be booting into Apple's servers and their Recovery HD so you can download and reinstall your factory assigned version of Lion. This process is called OS X Internet Recovery.
For example, my MBP Retina came preinsalled with Lion 10.7.4. When I use the process above, 10.7.4 gets downloaded and reinstalled.