When you get here you will automatically go into the cutscene in which you are given the merit badge for Telekinesis! You'll go into a training session again just as you did before in which he'll teach you how to use your new ability.
When you're done learning about your new ability you can use the UTS to go outside the main lodge, chances are good you don't have 800 arrowheads yet, so if you want to pick up the Cobweb Duster now get out your Dowsing Rod and start arrowhead hunting, it shouldn't take you too long to work up 800. After you have enough buy the duster and jump back in the UTS to return to Nein's lab.