How do you move boulders in underground base?

Talk to the underground man after completing his tutorial and he'll tell you he'll let you move boulders if you capture a flag. go into the underground and get a friend to go on his version and use the DS wireless link. find you're friend's secret base and talk to his PC it'll ask you if you want to steal his flag. take it and go to you're base. Talk to your PC and it'll register. You can now have 12 items in your base and remove 4-5 boulders. Hope it helped you.