Continue to follow the MSQ (the MSQ tracker should appear at the top left corner with your current/next available quest if your screen if you haven't changed your HUD) and you will naturally progress into each expansion.

The MSQ is a continuous story so you can't pick and choose what parts you do short of buying a story skip in the cash shop (not recommended as important story elements are scattered throughout the MSQ though certainly an option for those who don't care about story).