Leave Matt's flat and go to Emerson Express.
Give your boss the lingerie catalog. You'll get a package and delivery note.
Go to Matt's place. On the right side of the building is an entrance for SF convention. Talk with Murray, you need to show him a delivery note.
Go inside and on the left. Talk with Charlie. Randal doesn't have enough money to buy the ring.
Go back to the main hall and talk with Mel.
You can find there Randal's flatmate - Mortimer. That means you can search his room.