LEGO Rock Band

Enter Tour mode and go to your Rock Den. At any of the different room screens, press Yellow to enter the Customize Rock Den screen. Parts of the room will be highlighted (small wall, tall floor, etc). Select any of them and then scroll up or down until you find an item with a price. Buy it.

If it doesn't let you browse items then you haven't unlocked any items for that slot yet so try another one. You unlock more Rock Den items by playing gigs in the World Tour.

Just keep buying items and eventually this will unlock. If you want to get it fast then be sure to buy the Huge Floor items in various rooms and the many versions of the Speakers in the Practice Stage. These are the items with the highest costs.

I recommend going for this achievement later on in your Tour so you already have a lot of unlocked items to choose from and plenty of studs to spend.