You have to sign into your account that you were logged into, when you bought the expansion.
Next click on the desktop launcher icon and let it update the game, when it is done, the PLAY button will light up, and you will be able to play and level.
If you are still level 100, I suggest you do the sig line for Planes of Phophecy, to level to 120..
if you are higher the than 100, then here's the link to RoS.
Enjoy and happy grinding..