Wow I play transformice too :o Okay so what you do is you type: " /title " in the chat box. This brings up a list of all the titles you unlocked. Find the greedy mouse title along with its number. Now type: " /title 5". 5 is the number for Greedy Mouse. Your mouse should be wearing that title on the next map. Good luck :D
Once you unlock the titles, they will be displayed on your profile. You can use the title command to browse for your achieved title and you then apply it as your title on your profile page. You can go to transformmicewiki.com for more details too.