Follow this route:
Alt + F1 > System > Administration > Software Sources.
Then, from the tab "Linux Mint Software", enable "Unstable Packages (Romeo)" (just click in the box)
When you finish with this, run the Update Manager (from the panel, Notification Area or through System > Administration > Update Manager and that's it!
Sid Mint Xfce -32, Openbox, KDE, Enlightenment, Liquorix Kernel
HP Compaq Presario cq71, Intel Core 2 Duo T4400 @ 2.2 GHz, 3 GB DDR2 RAM, Nvidia Geforce G 103M